Add helper for dealing with compliance modes

Add a predicate to check whether we are in a given compliance mode, a
function that computes the minimum of the validity of all UIDs, and a
macro that can be used to check various objects for compliance with
CO_DE_VS without breaking compilation with older versions of GPGME,
and without cluttering the code with preprocessor conditionals.

• Signed-off-by: Justus Winter <justus@gnupg.org>