3.7: automatic annotation for vibrating square

Add "Gateway to Moloch's Sanctum" to the vibrating square level if you
step on the square or detect/magic map it as a pseudo-trap, an extra
hint for players who manage to get that far but then don't know what
to do next.  (I think I may also add a randomly placed floor engraving
along the lines of "For a good time, consult the Oracle of Delphi."
as a gag variant of "For a good time, call <name> at <phone number>."
Not very thematic for Gehennom but could conceivably nudge someone in
the right direction.  But it could give away the level for experienced
players who haven't located the vibrating square yet.)

The annotation sticks until the one for "Moloch's Sanctum" gets added.
That happens when the temple on the sanctum level is entered or the
altar there has become mapped (in view or via magic mapping).

Could break existing 3.7.0- save files (but probably won't, since
at least two bits were available unless using an ancient 'Bitfield()
allocates whole bytes' configuration).  That's the reason I didn't
put this into 3.6.2+.

include/dungeon.h

@@ -237,6 +237,13 @@ typedef struct mapseen {
            questing is for quest home (level 1) */
         Bitfield(quest_summons, 1); /* heard summons from leader */
         Bitfield(questing, 1); /* quest leader has unlocked quest stairs */
+        /* "gateway to sanctum" */
+        Bitfield(vibrating_square, 1); /* found vibrating square 'trap';
+                                        * flag cleared once the msanctum
+                                        * annotation has been added (on
+                                        * the next dungeon level; temple
+                                        * entered or high altar mapped) */
+        Bitfield(spare1, 1); /* not used */
     } flags;
     /* custom naming */
     char *custom;