add a RequiredRSASize for checking RSA key length in ssh(1).

add a RequiredRSASize for checking RSA key length in ssh(1).
User authentication keys that fall beneath this limit will be
ignored. If a host presents a host key beneath this limit then
the connection will be terminated (unfortunately there are no
fallbacks in the protocol for host authentication).

feedback deraadt, Dmitry Belyavskiy; ok markus@