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/vision.c

@@ -2133,7 +2133,7 @@ howmonseen(struct monst *mon)
     if (tp_sensemon(mon))
         how_seen |= MONSEEN_TELEPAT;
     /* xray */
-    if (useemon && xraydist > 0 && distu(mon->mx, mon->my) <= xraydist)
+    if (useemon && xraydist > 0 && mdistu(mon) <= xraydist)
         how_seen |= MONSEEN_XRAYVIS;
     /* extended detection */
     if (Detect_monsters)