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/sounds.c

@@ -1100,7 +1100,7 @@ dochat(void)
 
     if (!getdir("Talk to whom? (in what direction)")) {
         /* decided not to chat */
-        return ECMD_OK;
+        return ECMD_CANCEL;
     }
 
     if (u.usteed && u.dz > 0) {