docs: introduce "curldown" for libcurl man page format

curldown is this new file format for libcurl man pages. It is markdown
inspired with differences:

- Each file has a set of

docs: introduce "curldown" for libcurl man page format

curldown is this new file format for libcurl man pages. It is markdown
inspired with differences:

- Each file has a set of leading headers with meta-data
- Supports a small subset of markdown
- Uses .md file extensions for editors/IDE/GitHub to treat them nicely
- Generates man pages very similar to the previous ones
- Generates man pages that still convert nicely to HTML on the website
- Detects and highlights mentions of curl symbols automatically (when
their man page section is specified)

tools:

- cd2nroff: converts from curldown to nroff man page
- nroff2cd: convert an (old) nroff man page to curldown
- cdall: convert many nroff pages to curldown versions
- cd2cd: verifies and updates a curldown to latest curldown

This setup generates .3 versions of all the curldown versions at build time.

CI:

Since the documentation is now technically markdown in the eyes of many
things, the CI runs many more tests and checks on this documentation,
including proselint, link checkers and tests that make sure we capitalize the
first letter after a period...

Closes #12730

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tests: rename tests scripts to the test number

It is hard to name the scripts sensibly. Lots of them are similarly
named and the name did not tell which test that used them.

The

tests: rename tests scripts to the test number

It is hard to name the scripts sensibly. Lots of them are similarly
named and the name did not tell which test that used them.

The new approach is rather to name them based on the test number that
runs them. Also helps us see which scripts are for individual tests
rather than for general test infra.

- badsymbols.pl -> test1167.pl
- check-deprecated.pl -> test1222.pl
- check-translatable-options.pl -> test1544.pl
- disable-scan.pl -> test1165.pl
- error-codes.pl -> test1175.pl
- errorcodes.pl -> test1477.pl
- extern-scan.pl -> test1135.pl
- manpage-scan.pl -> test1139.pl
- manpage-syntax.pl -> test1173.pl
- markdown-uppercase.pl -> test1275.pl
- mem-include-scan.pl -> test1132.pl
- nroff-scan.pl -> test1140.pl
- option-check.pl -> test1276.pl
- options-scan.pl -> test971.pl
- symbol-scan.pl -> test1119.pl
- version-scan.pl -> test1177.pl

Closes #12487

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tests: remove leading spaces from some tags

The threee tags `<name>`, `</name>` and `<command>` were frequently used
with a leading space that this removes. The reason this habbit is so

tests: remove leading spaces from some tags

The threee tags `<name>`, `</name>` and `<command>` were frequently used
with a leading space that this removes. The reason this habbit is so
widespread in testcases is probably that they have been copy and pasted.

Hence, fixing them all now might curb this practice from now on.

Closes #12028

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version: add a feature names array to curl_version_info_data

Field feature_names contains a null-terminated sorted array of feature
names. Bitmask field features is deprecated.

version: add a feature names array to curl_version_info_data

Field feature_names contains a null-terminated sorted array of feature
names. Bitmask field features is deprecated.

Documentation is updated. Test 1177 and tests/version-scan.pl updated to
match new documentation format and extended to check feature names too.

Closes #9583

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test1177: look for curl.h in source directory

If we use a separate build directory, there is no copy of the header.

Closes #5310

test1177: verify that all the CURL_VERSION_ bits are documented