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/uinfo.cc

@@ -117,7 +117,7 @@ cygheap_user::init ()
 
    This needs careful checking should we use check_token_membership in other
    circumstances. */
-static bool
+bool
 check_token_membership (PSID sid)
 {
   NTSTATUS status;
@@ -142,7 +142,7 @@ check_token_membership (PSID sid)
   return false;
 }
 
-void
+static void
 internal_getlogin (cygheap_user &user)
 {
   struct passwd *pwd;