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/spell.c

@@ -302,7 +302,7 @@ deadbook(struct obj* book2)
                 && cansee(mtmp->mx, mtmp->my)) {
                 mtmp->mpeaceful = TRUE;
                 if (sgn(mtmp->data->maligntyp) == sgn(u.ualign.type)
-                    && distu(mtmp->mx, mtmp->my) < 4)
+                    && mdistu(mtmp) < 4)
                     if (mtmp->mtame) {
                         if (mtmp->mtame < 20)
                             mtmp->mtame++;