http: Allow overriding of the Host header.

* common/http.c (http_open): Add arg httphost.
(http_open_document): Pass NULL for httphost.
(send_request): Add arg httphost.  If given, use HTTPHOST instead of
SERVER.  Use https with a proxy if requested.
(http_verify_server_credentials): Do not stop at the first error
message.
* dirmngr/ocsp.c (do_ocsp_request): Adjust call to http_open.
* keyserver/curl-shim.c (curl_easy_perform): Ditto.
* dirmngr/ks-engine-http.c (ks_http_fetch): Ditto.
* dirmngr/ks-engine-hkp.c (ks_hkp_help): Ditto.
keyserver: Improve support for hkps pools.

* dirmngr/ks-engine-hkp.c (hostinfo_s): Add fields cname, v4addr, and
v6addr.
(create_new_hostinfo): Clear them.
(my_getnameinfo): Add args numeric and r_isnumeric.
(is_ip_address): New.
(map_host): Add arg r_host.  Rewrite the code to handle pools in a
special way.
(ks_hkp_print_hosttable): Change format of help info output.
(make_host_part): Add arg optional r_httphost.
(send_request): Add arg httphost.
(ks_hkp_search, ks_hkp_get, ks_hkp_put): Get httphost and pass it to
send_request.
--

This changes quite some things on how the hostinfo is maintained.
However, it might be better to rework the data structures and have one
entry per IP address instead of this clumsy patch.