Better handling of truncated messages.

Instrumented SSHD messages are currently limited to 4096 bytes, but
don't newline terminate messages that reach that limit.  We can
workaround that to ensure subsequent messages are actually treated
as distinct messages.

This also prevents the message buffer from becoming unbounded should
a client never send a newline.
Workaround missing channel_notty_analysis_disable_3 args.
Add final log message when stopping server.