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: T4393
- Signed-off-by: Daniel Kahn Gillmor <firstname.lastname@example.org>