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.

dirmngr/ks-engine-http.c

@@ -68,6 +68,7 @@ ks_http_fetch (ctrl_t ctrl, const char *url, estream_t *r_fp)
   err = http_open (&http,
                    HTTP_REQ_GET,
                    url,
+                   /* httphost */ NULL,
                    /* fixme: AUTH */ NULL,
                    0,
                    /* fixme: proxy*/ NULL,