Add kqueue support

Add kqueue support

* configure.ac (--with-file-notification): Add kqueue.
(top): Remove special test for "${HAVE_NS}" and
${with_file_notification}, this is handled inside gfilenotify
tests.  Add kqueue tests.  Use NOTIFY_CFLAGS and NOTIFY_LIBS
instead of library specific variables.

* src/Makefile.in: Use NOTIFY_CFLAGS and NOTIFY_LIBS.

* src/emacs.c (main): Call globals_of_kqueue and syms_of_kqueue.

* src/kqueue.c: New file.

* src/lisp.h: Declare extern globals_of_kqueue and syms_of_kqueue.