dirmngr: Align the gnutls use of CAs with the ntbtls code.

* dirmngr/http.c (http_session_new) <gnutls>: Use only the special
pool certificate for the default keyserver.

The gnutls version uses a different strategy than the ntbtls version
on when to use the special SKS pool certificate. This patch aligns it
so that we don't need to wonder about different kind of bug reports.
In short the special cert is now the only cert use with the default
keyserver.

• Signed-off-by: Werner Koch <wk@gnupg.org>