Previously, kqueue file monitor only add event sources for directories
regardless of the type of the file being monitored. Doing so may be
possible on inotify, but it is not sufficient on kqueue. Watching a
directory on kqueue doesn't report changes made to files under it, and
we must watch files themselves to get notified. This problem is fixed
by adding a second watch for non-directory file monitors, and the result
is that we are now able to receive 'CHANGED' and 'ATTRIBUTE_CHANGED'
events for non-directory files.

Since having two watches on one file monitor requires many code changes
to work properly, this commit also changes the following things:

 - NOTE_ALL macro is now replaced by note_all inline function. Since the
   kqueue backend is shared by all BSD operating systems, there are a
   few difference between these systems. It is easier to do '#ifdef'
   check in a function than in a macro.

 - Both g_kqueue_file_monitor_callback and g_kqueue_fi...