Import only user IDs matching address used for WKD lookup

When importing the keys retrieved with a WKD lookup, set an import filter
which filters out all user IDs that do not match the address that was
used for the WKD lookup. gpg does the same when importing keys via WKD.
This prevents the import of "forged" keys and user IDs distributed via WKD.

• GnuPG-bug-id: T5334