skip setting SO_REUSEADDR when socket is AF_UNIX

Summary:
After D38076789 (4a18fd1), we started to actually set `SO_REUSEADDR` on Windows. This has broken our build because we are using `folly::AsyncServerSocket` with a UNIX Domain Socket address on Windows. Setting `SO_REUSEADDR` on the socket confuses Windows and making it failed to bind.

It looks like we can safely skip setting `SO_REUSEADDR` when the address family is `AF_UNIX`. Neither Linux nor BSD supports it.

https://gavv.net/articles/unix-socket-reuse/
https://stackoverflow.com/a/15717204

Reviewed By: kvtsoy

Differential Revision: D38515166

fbshipit-source-id: 3dfc61ae2f5173e4750226f88fa6c32c50f76ee3

folly/io/async/AsyncServerSocket.cpp

@@ -829,8 +829,10 @@ void AsyncServerSocket::setupSocket(NetworkSocket fd, int family) {
 
   // Set reuseaddr to avoid 2MSL delay on server restart
   int one = 1;
-  if (netops::setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &one, sizeof(one)) !=
-      0) {
+  // AF_UNIX does not support SO_REUSEADDR, setting this would confuse Windows
+  if (family != AF_UNIX &&
+      netops::setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &one, sizeof(one)) !=
+          0) {
     auto errnoCopy = errno;
     // This isn't a fatal error; just log an error message and continue
     LOG(ERROR) << "failed to set SO_REUSEADDR on async server socket "