Refactoring Channels

1. Add flags for shell, sftp, scp, and exec to the wolfSSH connection
   tracker structure.
2. Add a channel request callback function that can tell if we are
   running a shell, SFTP, SCP, or trying to exec another command, and
   sets the flag in the connection tracker.
3. When processing the SFTP connection, and the stream peek returns 0,
   set a timeout for the select. (Stops it from spinning.)
4. The wolfSSH accept state machine is shorter now, it ends after user
   authentication, leaving the application to deal with new channels.
   Remove the handling of the return codes specifically for SFTP and
   SCP, as they are handled elsewhere.
5. Whitespace.