fix "unique monster doppelganger and bones bug"

Reported by paxed 8 years ago:  if a bones file contains a
doppelganger imitating a unique monster, when it gets loaded
that monster ends up being marked as having been created.  The
doppelganger itself will shapechange to other forms, but the
unique monster won't be created when it should be because it has
become extinct.

Report involved creating a statue of a unique monster which
yields a doppelganger in that monster's shape, then using stone
to flesh to animate the statue, dying before it changes to some
other shape, and having bones be saved.

src/restore.c

@@ -402,7 +402,9 @@ restmonchn(NHFILE* nhfp)
         offset = mtmp->mnum;
         mtmp->data = &mons[offset];
         if (ghostly) {
-            int mndx = monsndx(mtmp->data);
+            int mndx = (mtmp->cham == NON_PM) ? monsndx(mtmp->data)
+                                              : mtmp->cham;
+
             if (propagate(mndx, TRUE, ghostly) == 0) {
                 /* cookie to trigger purge in getbones() */
                 mtmp->mhpmax = DEFUNCT_MONSTER;