F: 'init' control messages

Each transport starts up with an 'init' message, which allows us to carry various pieces of information to the peer. Suggested by Jan Safranek, and ends up being very useful.

 * Protocol version checking
 * Use this to transfer the initial 'environment' we used to have in index.html
 * Transfer channel seeds so we don't have to do "Channel NAT" in CockpitWebService