* http.h, http.c (send_request): Pass in srvtag and make its presence
sufficient to turn the feature on.  (http_open): From here.
(http_document): And here.
* gpgkeys_hkp.c (srv_replace): New function to transform a SRV
hostname to a real hostname.  (main): Call it from here for the
HAVE_LIBCURL case (without libcurl is handled via the curl-shim).
* curl-shim.h, curl-shim.c (curl_easy_setopt, curl_easy_perform): Add
a CURLOPT_SRVTAG_GPG_HACK (passed through the the http engine).