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.