build: unix socket tidy-ups

- lib: delete two unused `<sys/un.h>` includes.

- lib: drop interim macro `WIN32_SOCKADDR_UN`.
  Follow-up to 0fe9018e1a #7737
  Also fixing a potential issue of leaving unix socket support disabled
  if any header would include Windows' `afunix.h`, and define
  `UNIX_PATH_MAX` on its own.

- connect: honor unix socket disable option.

- connect: simplify unix socket PP condition.
  `USE_UNIX_SOCKETS` already means the necessary header/type are
  available, guaranteed by configure. `AF_UNIX` is already used
  elsewhere in the code without explicit checks.

- curl_setup.h: document availability of `afunix.h` on Windows more.
  It requires mingw-w64 10+ or MS SDK 10.17763.0 VS2017 15.8+.

- curl_setup.h: use `afunix.h` with mingw-w64 v10+ to start avoiding
  the local workaround if possible.

- GHA/windows: test disable unix socket option on Windows.

Ref: https://devblogs.microsoft.com/commandline/af_unix-comes-to-windows/

Closes #19989

.github/workflows/windows.yml

@@ -429,7 +429,7 @@ jobs:
             env: 'x86_64'
             ver: '15.1.0'
             url: 'https://github.com/skeeto/w64devkit/releases/download/v2.2.0/w64devkit-x64-2.2.0.7z.exe'
-            config: '-DENABLE_DEBUG=ON -DBUILD_SHARED_LIBS=OFF -DCURL_USE_SCHANNEL=ON -DENABLE_UNICODE=OFF'
+            config: '-DENABLE_DEBUG=ON -DBUILD_SHARED_LIBS=OFF -DCURL_USE_SCHANNEL=ON -DENABLE_UNICODE=OFF -DENABLE_UNIX_SOCKETS=OFF'
             type: 'Release'
           - name: 'schannel'  # mingw-w64 10.0
             sys: 'mingw64'