dirmngr: Do not rewrite the redirection for the "openpgpkey" subdomain.

* dirmngr/http.c (same_host_p): Consider certain subdomains to be the
same.

(cherry picked from commit 37f0c55c7be3fc4912237f2bc72466aef6f8aa36)

• GnuPG-bug-id: T4603
• Signed-off-by: Werner Koch <wk@gnupg.org>