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.

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