Handle watching of several files in the same directory for inotify.

Handle watching of several files in the same directory for inotify.

Fixes: debbugs:18880
* filenotify.el (file-notify-descriptors, file-notify-handle-event):
Adapt docstring.
(file-notify--descriptor): New defun.
(file-notify-callback, file-notify-add-watch, file-notify-rm-watch):
Adapt docstring.  Handle multiple values for
`file-notify-descriptors' entries.

* net/tramp.el (tramp-handle-file-notify-rm-watch): Do not check
`file-notify-descriptors', the implementation has been changed.