ssh-agent: record failed session-bind attempts

ssh-agent: record failed session-bind attempts

Record failed attempts to session-bind a connection and refuse signing
operations on that connection henceforth.

Prevents a future situation where we add a new hostkey type that is not
recognised by an older ssh-agent, that consequently causes session-bind
to fail (this situation is only likely to arise when people mix ssh(1)
and ssh-agent(1) of different versions on the same host). Previously,
after such a failure the agent socket would be considered unbound and
not subject to restriction.

Spotted by Jann Horn