dirmngr: Fix handling of CNAMEed keyserver pools.

* dirmngr/ks-engine-hkp.c (map_host): Don't use the cname for HTTPHOST.
* dirmngr/server.c (make_keyserver_item): Map keys.gnupg.net.

For a description of the problem see the comment in
make_keyserver_item.

• GnuPG-bug-id: T3755
• Signed-off-by: Werner Koch <wk@gnupg.org>