knockback knocking riding hero out of saddle

I've implemented targetted dismount such that being knocked out of
the saddle will place the hero opposite the attacker in preference
to a random spot adjacent to the steed.  If that opposite spot
isn't appropriate, the two spots next to it get tried.

In these map fragments, H is knocking mounted hero off of u.  The
digits indicate priority of potential destinations.

|.....   |..21.
|...2.   |..u2.
|.Hu1.   |.H...
|...2.   |.....

If spot 1 isn't acceptable, both of spots 2 (in random order) will
be tried next.  If those aren't acceptable either, it will try the
other 5 spots adjacent to the steed (the one of those with the
attacker will always be unacceptable).  And as before, it none of
those work, it uses enexto() to pick a random spot as close to the
steed as feasible.

Not knockback:  when dismounting due to polymorph, avoid diagonal
adjacent spots if hero's new form can't move diagonally.  (The hero
can't already be in no-diagonal form because riding requires that
the rider be humanoid.  I keep thinking the restriction is "can't
be polymorphed" but that isn't correct.)

src/uhitm.c

@@ -4732,10 +4732,15 @@ mhitm_knockback(
 
     /* do the actual knockback effect */
     if (u_def) {
+        /* normally dx,dy indicates direction hero is throwing, zapping, &c
+           but here it is used to pass the preferred direction for dismount
+           to dismount_steed (used for DISMOUNT_KNOCKED only) */
+        u.dx = sgn(u.ux - magr->mx); /* [sgn() is superfluous here] */
+        u.dy = sgn(u.uy - magr->my); /* [ditto] */
         if (u.usteed)
             dismount_steed(DISMOUNT_KNOCKED);
         else
-            hurtle(u.ux - magr->mx, u.uy - magr->my, rnd(2), FALSE);
+            hurtle(u.dx, u.dy, rnd(2), FALSE);
 
         set_apparxy(magr); /* update magr's idea of where you are */
         if (!rn2(4))