Handler: Parser is required, even when not used

[KristianLyng]

I think we need a better way to verify handlers. It's rather pointless to require a parser that's never used, but by removing that requirement, the individual receivers need to verify this somehow. It's easy to do at run-time in Parse(), but that isn't exposed to Verify-logic, so ./skogul -show wont detect this, and besides, it adds a pointless op for every single parsed byte sequence.

Ideally I'd like this to be largely transparent to receivers, but that might not be possible.