gpg: Import stray revocation certificates.

* g10/kbnode.c (new_kbnode2): New.
* g10/import.c (delete_inv_parts): New arg r_otherrevsigs to store
misplaced revocations.
(import_revoke_cert): Allow to pass an entire list.
(import_one): Import revocations found by delete_inv_parts.

It might be useful to distribute revocations of old keys along with
new keys. This is in particicualrr useful for WKD stored keys. This
patch allows to put unrelated standalone revocations into a key. For
example they can simply appended to a keyblock. Right now it is a bit
inaesthetic to see diagnostics about misplaced or bad revocation
signatures.

Backported-from-master: 7aaedfb10767c74f3e6868dd1563cbbf1282ab2f