Support AF_UNIX listening sockets on Windows.

Not all Windows toolchains have this yet, so we have to put the
whole lot under #ifdef.

windows/platform.h

@@ -293,6 +293,7 @@ void socket_reselect_all(void);
 SockAddr *sk_namedpipe_addr(const char *pipename);
 /* Turn a WinSock error code into a string. */
 const char *winsock_error_string(int error);
+Socket *sk_newlistener_unix(const char *socketpath, Plug *plug);
 
 /*
  * network.c dynamically loads WinSock 2 or WinSock 1 depending on