common: Make the compliance check more robust.

* common/compliance.c (get_compliance_cache): New.
(gnupg_rng_is_compliant): Use per mode cache.
(gnupg_gcrypt_is_compliant): Ditto.

This addresses the problem tha the check might be called with
different compliance values and thus it should return the
corresponding cached result.

Backported from 2.3. Added PGP6 support which was removed from 2.3.

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