Because the session is activated on the *next* time through the
loop, if the third attempt to authenticate (including public key)
fails, it would leave the session inactive and fail to log in with
a confusing error about an obsolete cryptlib.

On success, decrement ssh_failed to ensure another pass through
the loop.

Also, add more debug logging around the auth so you can clearly
see each failure, and log client_socket each time so it's clearly
grouped.