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/apply.c

@@ -3288,8 +3288,7 @@ use_pole(struct obj *obj, boolean autohit)
     cc.y = u.uy;
     if (!find_poleable_mon(&cc, min_range, max_range) && hitm
         && !DEADMONSTER(hitm) && sensemon(hitm)
-        && distu(hitm->mx, hitm->my) <= max_range
-        && distu(hitm->mx, hitm->my) >= min_range) {
+        && mdistu(hitm) <= max_range && mdistu(hitm) >= min_range) {
         cc.x = hitm->mx;
         cc.y = hitm->my;
     }