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/zap.c

@@ -2380,7 +2380,7 @@ dozap(void)
         return ECMD_OK;
     obj = getobj("zap", zap_ok, GETOBJ_NOFLAGS);
     if (!obj)
-        return ECMD_OK;
+        return ECMD_CANCEL;
 
     check_unpaid(obj);