[PATCH v3 GnuPG] allow import without user ids

[Vincent Breitmoser]

I integrated one more of dkg's tests.  Subkey revocations are now imported even
in the absence of subkey binding certificates. I believe this patch series now
covers the low hanging fruit wrt import of partial keys.

This makes GnuPG fully compatible with the split identity/non-identity
information model of keys.openpgp.org, and fixes the issue mentioned in our FAQ:
https://keys.openpgp.org/about/faq#older-gnupg

Please let me know if there are any issues remaining with this patch, and I will
try to resolve them.