Impove "tunnelled protocols"

[iagox86]

Right now, I use flags in the SYN header for tunneled protocols (like ping, command, and listen (when I get it working).

I want to make the protocol easier to extend to tunnel other stuff that needs special handling on the server (say, socks5 or something). But using flags, I'll run out after 16 protocols, total, and I already have like 6. Super unsustainable!

My plan is to have a flag in the header for TUNNELED_PROTOCOL. If that's enabled, then there'll be another field (16 or 32 bits) indicating the tunneled protocol. That way, the client can only attempt one at a time (without crazy sanity checks) and the server will instantly know whether or not it's supported.

I'll likely use that to re-implement some of the stuff I already have, like "download" and "chunk" mode, which are pretty ugly right now (and removed from the client in my current branch because they're useless)