Add "user canceled" as extended command return value

Instead of returning ECMD_OK, the commands now return ECMD_CANCEL
when user declined to pick a direction or an object to act on.

Note that this can be ORed with ECMD_TIME, if the command still
took a turn.

For now this has no gameplay meaning.

src/dig.c

@@ -1016,7 +1016,7 @@ use_pick_axe(struct obj *obj)
     *dsp = 0;
     Sprintf(qbuf, "In what direction do you want to %s? [%s]", verb, dirsyms);
     if (!getdir(qbuf))
-        return res;
+        return (res|ECMD_CANCEL);
 
     return use_pick_axe2(obj);
 }