Use a common funcion for all monster healing

Previously, the code for monster healing was repeated every time it
was needed; this commit sends it all through a common function, which
will make it easier to make changes to how monster healing works in
the future.

This is just a code reorganisation and won't have any gameplay
effect unless I made a mistake.

src/do.c

@@ -876,7 +876,7 @@ engulfer_digests_food(struct obj *obj)
         } else if (could_grow) {
             (void) grow_up(u.ustuck, (struct monst *) 0);
         } else if (could_heal) {
-            u.ustuck->mhp = u.ustuck->mhpmax;
+            healmon(u.ustuck, u.ustuck->mhpmax, 0);
             /* False: don't realize that sight is cured from inside */
             mcureblindness(u.ustuck, FALSE);
         }