For HTTP, this refactors the existing logic, including the parsing of
the compression engine capability.
For SSH, this adds a ssh-only capability "protocaps" and a command for
informing the server on what the client supports. Since SSH is stateful,
keep track of the capabilities in the server instance.
This should be in the SSH Version 1 Transport section below, because I don't intent to carry this stateful feature forward to protocol version 2.
Also, the new capability should be documented in the capabilities section in this document.