In SSH packet logging mode, log SSH-2 packet sequence numbers, in

In SSH packet logging mode, log SSH-2 packet sequence numbers, in
both directions. We had a bug report yesterday about a Cisco router
sending SSH2_MSG_UNIMPLEMENTED and it wasn't clear for which packet;
logging the sequence numbers should make such problems much easier
to diagnose.

(In fact this logging fix wouldn't have helped in yesterday's case,
because the router also didn't bother to fill in the sequence number
field in the SSH2_MSG_UNIMPLEMENTED packet! This is a precautionary
measure against the next one of these problems.)

[originally from svn r8295]