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/mthrowu.c

@@ -826,7 +826,7 @@ spitmm(struct monst* mtmp, struct attack* mattk, struct monst* mtarg)
     struct obj *otmp;
 
     if (mtmp->mcan) {
-        if (!Deaf && distu(mtmp->mx, mtmp->my) < BOLT_LIM * BOLT_LIM) {
+        if (!Deaf && mdistu(mtmp) < BOLT_LIM * BOLT_LIM) {
             if (canspotmon(mtmp)) {
                 pline("A dry rattle comes from %s throat.",
                       s_suffix(mon_nam(mtmp)));