Add 'mdistu' macro

Short for distu(mtmp->mx, mtmp->my) (i.e. the distance between the hero
and the specified monster), which is a very common use of distu().  The
idea is that this would be a convenient shorthand for it; I actually
thought it (or something very similar) existed already, but couldn't
find it when I tried to use it earlier.  Based on the number of uses of
fully-spelled-out 'distu(mtmp->mx, mtmp->my)' replaced in this commit
I'm guessing I just imagined it.

src/pickup.c

@@ -2188,8 +2188,8 @@ reverse_loot(void)
                 if (coffers->spe == 2)
                     break; /* a throne room chest */
                 if (!otmp
-                    || distu(coffers->ox, coffers->oy)
-                           < distu(otmp->ox, otmp->oy))
+                    || (distu(coffers->ox, coffers->oy)
+                        < distu(otmp->ox, otmp->oy)))
                     otmp = coffers; /* remember closest ordinary chest */
             }
         if (!coffers)