Multiplexing packets based on protocol about uvgrtp

[jrsnen]

@altonen ok, thanks for the info. I didn't realize how this was implemented, so last year I added checks to handlers that discard packets that do not conform to said protocol, and this must be the problem with ZRTP. No need to revert any changes, just modifying those handlers (RTP and ZRTP), so they do nothing for wrong packets. I would have done this slightly differently, but the current implementation is fine.

Adding a way for users to get unprocessed packets would be nice still.

[altonen]

Multiplexing was supported at least before, that was the whole point of primary and auxiliary handlers of the packet dispatcher. I don't know if that functionality has since been removed but it probably can be resurrected from old commits.

[tampsa]

Implemented/fixed by efe7d98