Do something about `handler = true` in wupget handlers

Wupget handler interfaces under the same name have different signatures depending on `handler = true`, and anytime anybody has a problem it ALWAYS, EVERY TIME turns out to be `handler = true`. Design some alternative and put it in basecontent, then persuade major games to use it because that's where people copypaste from.

Start by reviewing the wupget handler interfaces and every `handler = true` variation referenced by the issue, then inspect the relevant material in basecontent. Define a concrete alternative that removes the signature ambiguity, place it in basecontent, and document how major games could adopt it.

Written by the indexing model from the issue text.