wkd: Restrict gpg-wks-client --mirror to the given domains.

* tools/gpg-wks-client.c (domain_matches_mbox): New.
(mirror_one_key): Skip non-matching domains.
(command_mirror): Change args to allow for several domains.

Although dirmngr returns only the keys matching a certain domain,
those keys still may have user ids from other domains. Now we publish
only the user-ids as specified on the command line.

• GnuPG-bug-id: T6224