Cygwin: split out fhandler_socket into inet and local classes

First cut, still incomplete

* fhandler_socket is now base class for other socket classes
* fhandler_socket_inet handles AF_INET and AF_INET6 sockets
* fhandler_socket_local handles AF_LOCAL/AF_UNIX sockets
* finally get rid of fdsock by using set_socket_handle in accept4
* align file-related calls (fstat,  fstatvfs, fchown, fchmod, facl)
  to Linux.

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

winsup/cygwin/security.h

@@ -17,6 +17,7 @@ details. */
 
 /* UID/GID */
 void uinfo_init ();
+bool check_token_membership (PSID);
 
 #define ILLEGAL_UID ((uid_t)-1)
 #define ILLEGAL_GID ((gid_t)-1)