Refactor the callback half of filenotify.el

Refactor the callback half of filenotify.el

Split callback code into backend-specific and general parts.  Refactor
pending event, which is always a rename, to include relevant
information only.  General clean-up.

* lisp/filenotify.el (file-notify--pending-event): Rename.
(file-notify--event-watched-file, file-notify--event-file-name)
(file-notify--event-file1-name, file-notify--event-cookie): Remove.
(file-notify--rename, file-notify--expand-file-name)
(file-notify--callback-inotify, file-notify--callback-kqueue)
(file-notify--callback-w32notify, file-notify--callback-gfilenotify)
(file-notify--call-handler, file-notify--handle-event): New.
(file-notify-callback): Split general parts into
file-notify--call-handler and file-notify--handle-event.
(file-notify--add-watch-inotify, file-notify--add-watch-kqueue)
(file-notify--add-watch-w32notify)
(file-notify--add-watch-gfilenotify): Use new callbacks.