github issue #1312 - prot from shape changers

Issue reported by youkan700:  shape change anomalies.  Shapechangers
could change shape despite active protection-from-shape-changers if
hero wore two rings of protection from shape changers and took one
off.  Shapechangers who migrated to a not-yet-visited level that
eventually got visited with protection from shape changers in effect
would be stuck in their current shape, even if the PfSC attribute
got toggled off and back on.

The issue included suggested fixes and those are what I've used.  I
noticed a third case that only applies to wizard mode:  if player
used #wizintrinsic to set a timed value for PfSC, monsters wouldn't
resume changing shape after it timed out, unless/until it got toggled
on and back off via a PfSC ring or hero left the level and returned.

Fixes #1312

src/dog.c

@@ -417,6 +417,12 @@ mon_arrive(struct monst *mtmp, int when)
     fromdlev.dnum = mtmp->mtrack[2].x;
     fromdlev.dlevel = mtmp->mtrack[2].y;
     mon_track_clear(mtmp);
+    /* in case Protection_from_shape_changers is different now from when
+       'mtmp' went onto the migrating monsters list; that's handled in
+       getlev() when returning to a previously visited level and by the
+       special level code for monsters specified in the level, but needed
+       here for monsters migrating to a newly created level */
+    restore_cham(mtmp);
 
     if (mtmp == u.usteed)
         return; /* don't place steed on the map */