* autorevert.el: Add file watch support.

* autorevert.el: Add file watch support.
(auto-revert-notify-enabled): New defconst.
(auto-revert-notify-watch-descriptor-hash-list)
(auto-revert-notify-watch-descriptor)
(auto-revert-notify-modified-p): New defvars.
(auto-revert-notify-rm-watch, auto-revert-notify-add-watch)
(auto-revert-notify-handler): New defuns.
(auto-revert-mode, global-auto-revert-mode): Remove file watches
when mode is disabled.
(auto-revert-handler): Check for `auto-revert-notify-modified-p'.
(auto-revert-buffers): Add file watches for active buffers.