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.

common/http.h

@@ -110,6 +110,7 @@ gpg_error_t http_raw_connect (http_t *r_hd,
 
 gpg_error_t http_open (http_t *r_hd, http_req_t reqtype,
                        const char *url,
+                       const char *httphost,
                        const char *auth,
                        unsigned int flags,
                        const char *proxy,