dirmngr: Use sks-keyservers CA by default for the hkps pool.

* dirmngr/Makefile.am (dist_pkgdata_DATA): Add sks-keyservers.netCA.pem.
* dirmngr/http.c (http_session_new): Add optional arg
intended_hostname and set a default cert.
* dirmngr/ks-engine-hkp.c (send_request): Pass httphost to
http_session_new.

Ship the certificate for the sks-keyservers hkps pool. If the user
has specified that they want to use
hkps://hkps.pool.sks-keyservers.net, and they have not specified any
hkp-cacert explicitly, then initialize the trust path with this
specific trust anchor.

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