RSA key configuration limitations

[Dmitry Belyavskiy]

Dear colleagues,

There is a need to increase RSA key requirements to make the installations
more secure. Just updating the default compiled-in value isn't an option
because it may significantly break legacy systems compatibility. This PR
[1] introduces a new configuration option MinRSABits to be managed for
security's sake.

If this approach is OK for upstream, please let me know and I will improve
this PR according to the feedback.

[1] https://github.com/openssh/openssh-portable/pull/325

-- 
Dmitry Belyavskiy