Shapeshifters change shape less

Non-vampire shapeshifters were changing shape a lot; often you
could have a chameleon turning into a different monster each turn.

Now monster shape changing is actually their "special" power,
and it takes 3-12 turns until they can do it again.

doc/fixes3-7-0.txt

@@ -1595,6 +1595,7 @@ monster priests and wizards did not cast spells
 prevent phaseable monsters hiding deep inside nondiggable walls
 blessed potion of see invisible does not guarantee the intrinsic
 prevent selecting all options in #optionsfull menu
+shapeshifters change shape less
 
 
 Fixes to 3.7.0-x General Problems Exposed Via git Repository

src/mon.c

@@ -4878,8 +4878,10 @@ decide_to_shapeshift(struct monst *mon)
 
     if (!is_vampshifter(mon)) {
         /* regular shapeshifter; 'ptr' is Null */
-        if (!rn2(6))
+        if (!mon->mspec_used && !rn2(6)) {
             dochng = TRUE;
+            mon->mspec_used = 3 + rn2(10);
+        }
     } else if (!(mon->mstrategy & STRAT_WAITFORU)) {
         /* The vampire has to be in good health (mhp) to maintain
          * its shifted form.