handle gpgv's weak-digests ERRSIG

handle gpgv's weak-digests ERRSIG

Our own gpgv method can declare a digest algorithm as untrusted and
handles these as worthless signatures. If gpgv comes with inbuilt
untrusted (which is called weak in official terminology) which it e.g.
does for MD5 in recent versions we should handle it in the same way.

To check this we use the most uncommon still fully trusted hash as a
configureable one via a hidden config option to toggle through all of
the three states a hash can be in.