rsa: Add FIPS 186-4 compliant RSA probable prime key generator.

* cipher/primegen.c (_gcry_fips186_4_prime_check): New.
* cipher/rsa.c (generate_fips): New.
(rsa_generate): Use new function in fips mode or with test-parms.
* tests/keygen.c (check_rsa_keys): Add test using e=65539.

Tomáš's patch war originally for libgcrypt 1.6.3 and has been ported
to master (1.7) by wk. Further changes:

• ChangeLog entries.
• Some re-indentation
• Use an extra test case instead of changing an existing one.

• Signed-off-by: Werner Koch <wk@gnupg.org>