add a "Match invalid-user" predicate to sshd_config Match options.

add a "Match invalid-user" predicate to sshd_config Match options.

This allows writing Match conditions that trigger for invalid username.
E.g.

PerSourcePenalties refuseconnection:90s
Match invalid-user
  RefuseConnection yes

Will effectively penalise bots try to guess passwords for bogus accounts,
at the cost of implicitly revealing which accounts are invalid.

feedback markus@