Cygwin: AF_LOCAL: allow opening with the O_PATH flag

If that flag is not set, or if an attempt is made to open a different
type of socket, the errno is now EOPNOTSUPP instead of ENXIO.  This is
consistent with POSIX, starting with the 2016 edition.  Earlier
editions were silent on this issue.

Opening is done in a (new) fhandler_socket_local::open method by
calling fhandler_base::open_fs.

Also add a corresponding fhandler_socket_local::close method.

winsup/cygwin/fhandler_socket_local.cc

@@ -634,6 +634,26 @@ fhandler_socket_local::dup (fhandler_base *child, int flags)
   return fhandler_socket_wsock::dup (child, flags);
 }
 
+int
+fhandler_socket_local::open (int flags, mode_t mode)
+{
+  /* We don't support opening sockets unless O_PATH is specified. */
+  if (flags & O_PATH)
+    return open_fs (flags, mode);
+
+  set_errno (EOPNOTSUPP);
+  return 0;
+}
+
+int
+fhandler_socket_local::close ()
+{
+  if (get_flags () & O_PATH)
+    return fhandler_base::close ();
+  else
+    return fhandler_socket_wsock::close ();
+}
+
 int __reg2
 fhandler_socket_local::fstat (struct stat *buf)
 {