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/do.c

@@ -2035,7 +2035,7 @@ revive_corpse(struct obj *corpse)
                     pline("%s claws itself out of the ground!",
                           canspotmon(mtmp) ? Amonnam(mtmp) : Something);
                     newsym(mtmp->mx, mtmp->my);
-                } else if (distu(mtmp->mx, mtmp->my) < 5*5)
+                } else if (mdistu(mtmp) < 5*5)
                     You_hear("scratching noises.");
                 break;
             }