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/eat.c

@@ -2817,7 +2817,7 @@ use_tin_opener(struct obj *obj)
 
     otmp = getobj("open", tinopen_ok, GETOBJ_NOFLAGS);
     if (!otmp)
-        return res;
+        return (res|ECMD_CANCEL);
 
     start_tin(otmp);
     return ECMD_TIME;