gpg: Fix --rebuild-keydb-caches.

* g10/parse-packet.c (parse_key): Store even unsupported packet
versions.
* g10/keyring.c (keyring_rebuild_cache): Do not copy keys with
versions less than 4.

That function, which is implicitly called while checking the keydb, led
to corruption of v3 key packets in the keyring which would later spit
out "packet(6)too short" messages.

• Signed-off-by: Werner Koch <wk@gnupg.org>