tests: make the unit test result type `CURLcode`

Before this patch, the result code was a mixture of `int` and
`CURLcode`.

Also adjust casts and fix a couple of minor issues fou

tests: make the unit test result type `CURLcode`

Before this patch, the result code was a mixture of `int` and
`CURLcode`.

Also adjust casts and fix a couple of minor issues found along the way.

Cherry-picked from #13489
Closes #13600

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windows: delete redundant headers

`winsock2.h` pulls in `windows.h`. `ws2tcpip.h` pulls in `winsock2.h`.
`winsock2.h` and `ws2tcpip.h` are also pulled by `curl/curl.h`.

Keep onl

windows: delete redundant headers

`winsock2.h` pulls in `windows.h`. `ws2tcpip.h` pulls in `winsock2.h`.
`winsock2.h` and `ws2tcpip.h` are also pulled by `curl/curl.h`.

Keep only those headers that are not already included, or the code under
it uses something from that specific header.

Closes #12539

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windows: use built-in `_WIN32` macro to detect Windows

Windows compilers define `_WIN32` automatically. Windows SDK headers
or build env defines `WIN32`, or we have to take care of it. T

windows: use built-in `_WIN32` macro to detect Windows

Windows compilers define `_WIN32` automatically. Windows SDK headers
or build env defines `WIN32`, or we have to take care of it. The
agreement seems to be that `_WIN32` is the preferred practice here.
Make the source code rely on that to detect we're building for Windows.

Public `curl.h` was using `WIN32`, `__WIN32__` and `CURL_WIN32` for
Windows detection, next to the official `_WIN32`. After this patch it
only uses `_WIN32` for this. Also, make it stop defining `CURL_WIN32`.

There is a slight chance these break compatibility with Windows
compilers that fail to define `_WIN32`. I'm not aware of any obsolete
or modern compiler affected, but in case there is one, one possible
solution is to define this macro manually.

grepping for `WIN32` remains useful to discover Windows-specific code.

Also:

- extend `checksrc` to ensure we're not using `WIN32` anymore.

- apply minor formatting here and there.

- delete unnecessary checks for `!MSDOS` when `_WIN32` is present.

Co-authored-by: Jay Satiro
Reviewed-by: Daniel Stenberg

Closes #12376

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test1960: verify CURL_SOCKOPT_ALREADY_CONNECTED

When returned from the CURLOPT_SOCKOPTFUNCTION, like when we have a
custom socket connected in the app, passed in to libcurl.

Ver

test1960: verify CURL_SOCKOPT_ALREADY_CONNECTED

When returned from the CURLOPT_SOCKOPTFUNCTION, like when we have a
custom socket connected in the app, passed in to libcurl.

Verifies the fix in #10648

Closes #10651

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