gpg: allow import of previously known keys, even without UIDs

gpg: allow import of previously known keys, even without UIDs

* g10/import.c (import_one): Accept an incoming OpenPGP certificate that
has no user id, as long as we already have a local variant of the cert
that matches the primary key.

--

This fixes two of the three broken tests in import-incomplete.scm.

GnuPG-Bug-id: 4393
Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
Gbp-Pq: Topic import-merge-without-userid
Gbp-Pq: Name gpg-allow-import-of-previously-known-keys-even-without-UI.patch