improve quasi-documentation of notify_status values

the sql in for instance includes/functions_posting.php at l1237, l1349, l1358 contains magic numbers for the notify_status column of forums_watch table, it would be nicer to implement at least define'd constants such as NO_NOTIFY, NOTIFY_YES, for legibility; there is a forum post that refers to the correct values, as well of course as the wiki; however given that [as well as the possibly counter-intuitive fact that 0=notify, 1=no notify] define'd constants are generally implemented in phpBB to avoid magic numbers it would be an improvement to replace the magic numbers; this may affect additional tables / notifications too