Add AF_UNIX support on windows

[p1-0tr]

Add support for using unix domain sockets on Windows.

[yhirose]

@p1-0tr could you please add unit tests? Then, I'll start reviewing. Thanks!

[p1-0tr]

@yhirose I enabled the existing test. Only the basic case, because windows only has support for basic features on AF_UNIX (no fd passing, or other anciliary data, autobind, socketpair, etc. - https://devblogs.microsoft.com/commandline/af_unix-comes-to-windows/#unsupported\unavailable: )

[yhirose]

@p1-0tr I just reviewed the code, but the condition looks a bit complicated. Can it be simpler as shown below?

      if (socket_options) { socket_options(sock); }

#ifdef _WIN32
      // Setting SO_REUSEADDR seems not to work well with AF_UNIX on windows, so
      // avoid setting default_socket_options.
      detail::set_socket_opt(sock, SOL_SOCKET, SO_REUSEADDR, 0);
#endif

[p1-0tr]

@yhirose Yes, that works.

I went with a comparison on the settings callback as I thought it would be more in line with the principle of least surprise (from the user's point of view). But I am happy with the simpler approach :)

[yhirose]

@p1-0tr thanks for the contribution!

[yhirose]

@p1-0tr could you take a look at this error on GitHub Actions CI?

https://github.com/yhirose/cpp-httplib/actions/runs/14047884306/job/39343816422

[p1-0tr]

@yhirose I created a fix for it - #2118 . Unfortunately ordering of winsock2.h and afunix.h includes matters.