overriding message suppression, revisited

This is more robust than the previous hack.  The issue of whether to
use it in other places is still unexplored.  Ultimately it's the user's
fault if overzealous message suppression hides something important.
[For an eerie game, try 'MSGTYPE=hide .'.]

include/decl.h

@@ -406,6 +406,8 @@ struct plinemsg_type {
 #define MSGTYP_NOREP    1
 #define MSGTYP_NOSHOW   2
 #define MSGTYP_STOP     3
+/* bitmask for callers of hide_unhide_msgtypes() */
+#define MSGTYP_MASK_REP_SHOW ((1 << MSGTYP_NOREP) | (1 << MSGTYP_NOSHOW))
 
 E struct plinemsg_type *plinemsg_types;