This change revamps SSH banner parsing. The previous behavior was both
a bit too strict in some regards, and too permissive in other.
Specifically, clients are now required to send a line starting with
"SSH-" as the first line. This is in line with the RFC, as well with
observed behavior. This also prevents the creation of `ssh.log` for
non-SSH traffic on port 22.
For the server side, we now accept text before the SSH banner. This
previously led to a protocol violation but is allowed by the spec.
New tests are added to cover these cases.
