Issue 1447: Pass proper Host header and SNI when SRV is used with curl.
* configure.ac: Check for inet_ntop. * m4/libcurl.m4: Provide a #define for the version of the curl library. * keyserver/gpgkeys_hkp.c (main, srv_replace): Call getaddrinfo() on each target. Once we find one that resolves to an address (whether IPv4 or IPv6), pass it into libcurl via CURLOPT_RESOLVE using the SRV name as the "host". Force the HTTP Host header to be the same.