Fixed `forset` backwards compatibility

[ohlidalp]

Fixes #3053

Reprint of code comments about forset parsing:

    // --------------------------------------------------------------------------------------------
    // BEWARE OF QUIRKS in the following code (they must be preserved for backwards compatibility):
    // - a space between the 'forset' keyword and arguments is optional.
    // - garbage characters anywhere on the line will silently add node 0 to the set. <<<========= THE NEW ONE!
    // - a separator at the end of line will silently add node 0 to the set.
    // --------------------------------------------------------------------------------------------