ssh: Returned faked response for the new session-bind extension.

* agent/command-ssh.c (SSH_OPT_CONSTRAIN_MAXSIGN): New.
(SSH_OPT_CONSTRAIN_EXTENSION): New.
(ssh_handler_add_identity): Ignore them.
(ssh_handler_extension): Take success for session-bind.

OpenSSH 8.9 does not gracefully allow communication with older agent
implementations. Until this new OpenSSH feature has been settled we
return a faked response.

Code has not yet been tested.

• GnuPG-bug-id: T5931