Changing Channels

[ejohnstown]

This set of commits changes how the server code handles a connection accept and channel opening. Originally, opening a channel is part of the accept state machine. The server code now completes the accept after user authentication. This PR introduces a set of callback function hooks for channel opening, end-of-file, and closing, success and failure, as well as various channel requests. On top of this, the echoserver and wolfSSHd are updated to take advantage of the new callbacks.