fix #Q382 - jumping possible while riding a sleeping pet (trunk only)

From a bug report:  when riding,
#jump command and jumping spell didn't check whether steed is able to move.
This makes #jump command, either magic boots or knight's innate ability,
fail if the steed can't move.  Spell still succeeds; its magic overrides
steed's immobility.  Fix is trunk only since it relies on a function that
isn't in the branch.

     Also, tweak hero movement to avoid an extra function call per move
when not riding.

src/apply.c

@@ -1332,6 +1332,12 @@ int magic; /* 0=Physical, otherwise skill level */
 	} else if (!magic && !Jumping) {
 		You_cant("jump very far.");
 		return 0;
+#ifdef STEED
+	/* if steed is immobile, can't do physical jump but can do spell one */
+	} else if (!magic && u.usteed && stucksteed(FALSE)) {
+		/* stucksteed gave "<steed> won't move" message */
+		return 0;
+#endif
 	} else if (u.uswallow) {
 		if (magic) {
 			You("bounce around a little.");