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/weapon.c

@@ -863,9 +863,7 @@ mon_wield_item(struct monst *mon)
             /* 3.6.3: artifact might be getting wielded by invisible monst */
             else if (cansee(mon->mx, mon->my))
                 pline("Light begins shining %s.",
-                      (distu(mon->mx, mon->my) <= 5 * 5)
-                          ? "nearby"
-                          : "in the distance");
+                      (mdistu(mon) <= 5 * 5) ? "nearby" : "in the distance");
         }
         obj->owornmask = W_WEP;
         return 1;