RT992: RSA_check_key should have a callback arg

The original RT request included a patch.  By the time
we got around to doing it, however, the callback scheme
had changed. So I wrote a new function RSA_check_key_ex()
that uses the BN_GENCB callback.  But thanks very much
to Vinet Sharma <vineet.sharma@gmail.com> for the
initial implementation.

Reviewed-by: Dr. Stephen Henson <steve@openssl.org>