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/pray.c

@@ -2192,7 +2192,7 @@ doturn(void)
            #turn operating through walls, not to require that the hero be
            able to see the target location */
         if (!couldsee(mtmp->mx, mtmp->my)
-            || distu(mtmp->mx, mtmp->my) > range)
+            || mdistu(mtmp) > range)
             continue;
 
         if (!mtmp->mpeaceful