Cygwin: AF_UNIX: store per-socket info in shared memory

Per-socket info in fhandler isn't correctly shared between multiple
instances of th same descriptor.  Implement a basic shared info which
is shared between all instances of a socket.

This also requires to move the fhandler_socket status bits into
fhandler_socket_wsock since the data is moved to the shared region
for AF_UNIX sockets.

Also, drop backing file requirement for socketpair server socket.
This will be handled differently in recvmsg/sendmsg.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/fhandler_socket.cc

@@ -31,8 +31,7 @@ fhandler_socket::fhandler_socket () :
   gid (myself->gid),
   mode (S_IFSOCK | S_IRWXU | S_IRWXG | S_IRWXO),
   _rcvtimeo (INFINITE),
-  _sndtimeo (INFINITE),
-  status ()
+  _sndtimeo (INFINITE)
 {
 }