RSA security bits calculation

RSA security bits calculation

NIST has updated their guidelines in appendix D of SP 800-56B rev2 (draft)
providing a formula for the number of security bits it terms of the length
of the RSA key.

This is an implementation of this formula using fixed point arithmetic.
For integers 1 .. 100,000 it rounds down to the next smaller 8 bit strength
270 times.  It never errs to the high side.  None of the rounded values occur
near any of the commonly selected lengths.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7352)