Channel init responses should answer a challenge

[mchlnix]

Right now they are just filled with random bytes and can easily be faked and sent to the client.

[mchlnix]

At least we can fill it with 0s and have that as a check for the client.

[mchlnix]

They have a specific message type byte in the header, which is protected by the link key, so they're not that easy to fake, but maybe there could still be more to it in the future.