lib: Curl_read/Curl_write clarifications

- replace `Curl_read()`, `Curl_write()` and `Curl_nwrite()` to
clarify when and at what level they operate
- send/recv of transfer related

lib: Curl_read/Curl_write clarifications

- replace `Curl_read()`, `Curl_write()` and `Curl_nwrite()` to
clarify when and at what level they operate
- send/recv of transfer related data is now done via
`Curl_xfer_send()/Curl_xfer_recv()` which no longer has
socket/socketindex as parameter. It decides on the transfer
setup of `conn->sockfd` and `conn->writesockfd` on which
connection filter chain to operate.
- send/recv on a specific connection filter chain is done via
`Curl_conn_send()/Curl_conn_recv()` which get the socket index
as parameter.
- rename `Curl_setup_transfer()` to `Curl_xfer_setup()` for
naming consistency
- clarify that the special CURLE_AGAIN hangling to return
`CURLE_OK` with length 0 only applies to `Curl_xfer_send()`
and CURLE_AGAIN is returned by all other send() variants.
- fix a bug in websocket `curl_ws_recv()` that mixed up data
when it arrived in more than a single chunk (to be made
into a sperate PR, also)

Added as documented [in
CLIENT-READER.md](https://github.com/curl/curl/blob/5b1f31dfbab8aef467c419c68aa06dc738cb75d4/docs/CLIENT-READERS.md).

- old `Curl_buffer_send()` completely replaced by new `Curl_req_send()`
- old `Curl_fillreadbuffer()` replaced with `Curl_client_read()`
- HTTP chunked uploads are now formatted in a client reader added when
needed.
- FTP line-end conversions are done in a client reader added when
needed.
- when sending requests headers, remaining buffer space is filled with
body data for sending in "one go". This is independent of the request
body size. Resolves #12938 as now small and large requests have the
same code path.

Changes done to test cases:

- test513: now fails before sending request headers as this initial
"client read" triggers the setup fault. Behaves now the same as in
hyper build
- test547, test555, test1620: fix the length check in the lib code to
only fail for reads *smaller* than expected. This was a bug in the
test code that never triggered in the old implementation.

Closes #12969

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bufq: remove Curl_bufq_skip_and_shift (unused)

Closes #11915

lib: remove unused functions, make single-use static

Closes #11174

http2: support HTTP/2 to forward proxies, non-tunneling

- with `--proxy-http2` allow h2 ALPN negotiation to
forward proxies
- applies to http: requests against a https: proxy only,

http2: support HTTP/2 to forward proxies, non-tunneling

- with `--proxy-http2` allow h2 ALPN negotiation to
forward proxies
- applies to http: requests against a https: proxy only,
as https: requests will auto-tunnel
- adding a HTTP/1 request parser in http1.c
- removed h2h3.c
- using new request parser in nghttp2 and all h3 backends
- adding test 2603 for request parser
- adding h2 proxy test cases to test_10_*

scorecard.py: request scoring accidentally always run curl
with '-v'. Removed that, expect double numbers.

labeller: added http1.* and h2-proxy sources to detection

Closes #10967

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http3: improvements across backends

- ngtcp2: using bufq for recv stream data
- internal stream_ctx instead of `struct HTTP` members
for quiche, ngtcp2 and msh3
- no more QUIC

http3: improvements across backends

- ngtcp2: using bufq for recv stream data
- internal stream_ctx instead of `struct HTTP` members
for quiche, ngtcp2 and msh3
- no more QUIC related members in `struct HTTP`
- experimental use of recvmmsg(), disabled by default
- testing on my old debian box shows no throughput improvements.
- leaving it in, but disabled, for future revisit
- vquic: common UDP receive code for ngtcp2 and quiche
- vquic: common UDP send code for ngtcp2 and quiche
- added pytest skips for known msh3 failures
- fix unit2601 to survive torture testing
- quiche: using latest `master` from quiche and enabling large download
tests, now that key change is supported
- fixing test_07_21 where retry handling of starting a stream
was faulty
- msh3: use bufq for recv buffering headers and data
- msh3: replace fprintf debug logging with LOG_CF where possible
- msh3: force QUIC expire timers on recv/send to have more than
1 request per second served

Closes #10772

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http2: flow control and buffer improvements

- use bufq for send/receive of network data
- usd bufq for send/receive of stream data
- use HTTP/2 flow control with no-auto updates to c

http2: flow control and buffer improvements

- use bufq for send/receive of network data
- usd bufq for send/receive of stream data
- use HTTP/2 flow control with no-auto updates to control the
amount of data we are buffering for a stream
HTTP/2 stream window set to 128K after local tests, defined
code constant for now
- elminiating PAUSEing nghttp2 processing when receiving data
since a stream can now take in all DATA nghttp2 forwards

Improved scorecard and adjuste http2 stream window sizes
- scorecard improved output formatting and options default
- scorecard now also benchmarks small requests / second

Closes #10771

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lib: add `bufq` and `dynhds`

Adding `bufq`:
- at init() time configured to hold up to `n` chunks of `m` bytes each.
- various methods for reading from and writing to it.
- `peek`

lib: add `bufq` and `dynhds`

Adding `bufq`:
- at init() time configured to hold up to `n` chunks of `m` bytes each.
- various methods for reading from and writing to it.
- `peek` support to get access to buffered data without copy
- `pass` support to allow buffer flushing on write if it becomes full
- use case: IO buffers for dynamic reads and writes that do not blow up
- distinct from `dynbuf` in that:
- it maintains a read position
- writes on a full bufq return CURLE_AGAIN instead of nuking itself
- Init options:
- SOFT_LIMIT: allow writes into a full bufq
- NO_SPARES: free empty chunks right away
- a `bufc_pool` that can keep a number of spare chunks to
be shared between different `bufq` instances

Adding `dynhds`:
- a straightforward list of name+value pairs as used for HTTP headers
- headers can be appended dynamically
- headers can be removed again
- headers can be replaced
- headers can be looked up
- http/1.1 formatting into a `dynbuf`
- configured at init() with limits on header counts and total string
sizes
- use case: pass a HTTP request or response around without being version
specific
- express a HTTP request without a curl easy handle (used in h2 proxy
tunnels)
- future extension possibilities:
- conversions of `dynhds` to nghttp2/nghttp3 name+value arrays

Closes #10720

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