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

lib/cf-socket.c

@@ -27,9 +27,6 @@
 #ifdef HAVE_NETINET_IN_H
 #include <netinet/in.h> /* <netinet/tcp.h> may need it */
 #endif
-#ifdef HAVE_SYS_UN_H
-#include <sys/un.h> /* for sockaddr_un */
-#endif
 #ifdef HAVE_LINUX_TCP_H
 #include <linux/tcp.h>
 #elif defined(HAVE_NETINET_TCP_H)