Support filesystem notification through inotify on GNU/Linux.

Support filesystem notification through inotify on GNU/Linux.

 configure.ac (inotify): New option.
 (HAVE_INOTIFY): Test for inotify.

 src/termhooks.h (enum event_kind) [HAVE_INOTIFY]: Add
 FILE_NOTIFY_EVENT.
 src/lisp.h (syms_of_inotify) [HAVE_INOTIFY]: Add prototype.
 src/keyboard.c (Qfile_inotify) [HAVE_INOTIFY]: New variable.
 (syms_of_keyboard): DEFSYM it.
 (kbd_buffer_get_event) [HAVE_INOTIFY]: Generate FILE_NOTIFY_EVENT.
 (make_lispy_event): Support FILE_NOTIFY_EVENT by generating
 Qfile_inotify events.
 (keys_of_keyboard) [HAVE_INOTIFY]: Bind file-inotify events in
 special-event-map to inotify-handle-event.
 src/emacs.c (main) [HAVE_INOTIFY]: Call syms_of_inotify.
 src/Makefile.in (base_obj): Add inotify.o.
 src/inotify.c: New file.

 lisp/subr.el (inotify-event-p, inotify-handle-event): New functions.

 test/automated/inotify-test.el: New test.