upstream: Add an ssh -Oconninfo command

upstream: Add an ssh -Oconninfo command

that shows connection information, similar to the ~I escapechar.
This is the first use of the mux extension mechanism, so it should be
both forward and backward compatible: a new client talking to an old
server will not allow the "conninfo" request to be sent, but everything
else should work seamlessly.  feedback and ok djm@

OpenBSD-Commit-ID: 50f047a85da277360558cabdfed59cb66f754341