Implement gfile-valid-p

Implement gfile-valid-p

* lisp/filenotify.el (file-notify-callback): Fix typo.
(gfile-valid-p): Remove defalias.

* src/gfilenotify.c (dir_monitor_callback): Cancel the monitor if
the file or directory to be watched is deleted.
(Fgfile_add_watch): Make watch_object a triple.
(Fgfile_rm_watch): Check, whether watch is cancelled already.
(Fgfile_valid_p): New defun.
(syms_of_gfilenotify): Declare Sgfile_valid_p.