Add support for AF_UNIX sockets

Windows 10 received support for AF_UNIX sockets in Insider Build 17063. This merge request adds basic support for AF_UNIX sockets to ws2_32 and wineserver.

Of particular note is the difficulty in handling sun_path. Most of the functions that allow for translating Windows paths to Unix paths are not accessible from ws2_32. I considered the following options:

• Pass the Windows path to wineserver and do the conversion there.
  • This is, as far as I can tell, not possible without major rearchitecting. wineserver does not have functions to translate Windows paths to Unix paths, for obvious reasons.
• Obtain the current working directory of the requesting process and temporarily change directories to there.
  • This only handles relative paths and fails for absolute paths, UNC paths, etc.
• Conditionally change directories based on whether the path is relative or not.
  • This is error-prone and wineserver does not have the requisite functions to do this cleanly.

I ultimately decided to pass the translated Unix path to wineserver, which changes directories to dirname(path). It then provides bind and connect with basename(path). This is not threadsafe, but wineserver is not (currently) multithreaded.

Abstract sockets are not fully supported by this patch, matching the behavior of Windows.