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/mcastu.c

@@ -363,8 +363,7 @@ m_cure_self(struct monst *mtmp, int dmg)
         if (canseemon(mtmp))
             pline_mon(mtmp, "%s looks better.", Monnam(mtmp));
         /* note: player healing does 6d4; this used to do 1d8 */
-        if ((mtmp->mhp += d(3, 6)) > mtmp->mhpmax)
-            mtmp->mhp = mtmp->mhpmax;
+        healmon(mtmp, d(3, 6), 0);
         dmg = 0;
     }
     return dmg;