gpg: Exclude expired trusted keys from the key validation process.

* g10/trustdb.c (copy_key_item): New.
(validate_keys): Use a stripped down UTK list w/o expired keys.

This patch makes sure that an expired trusted key is not used for
trust computation. The test case is to delete a trusted key from the
keyring, import a copy of that key which has already expired, check
that a signed key is not anymore fully trusted and finally import a
prolonged version of the trusted key and check that the signed key is
now again fully trusted.

• GnuPG-bug-id: T7200