Reduce ifdef soup by adding pre-3.0 compat support

This change creates a static inline function, BN_check_prime, for
pre-3.0 use which is implemented with the previous (1.1) compatible call
under the covers, BN_is_prime_ex.

The nchecks parameter value is maintained, even though it has no
noticable behavior change, given that the documentation clearly states
that at least 64 or 128 rounds are executed on the backend, depending on
how many bits there are in the given number being factored out.

MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D40305