Woops, partly revert my previous commit and do it slightly differently instead.

The signalling of that a global DNS cache is wanted is done by setting the
option but the setting of the internal variable that it is in use must not be
done until it finally actually gets used!

NOTE and WARNING: I noticed that you can't actually switch off the global dns
cache with CURLOPT_DNS_USE_GLOBAL_CACHE but you couldn't do that previously
either and the option is very clearly and loudly documented as DO NOTE USE so
I won't bother to fix this bug now.

lib/urldata.h

@@ -1443,6 +1443,7 @@ struct UserDefined {
   curl_ftpauth ftpsslauth; /* what AUTH XXX to be attempted */
   curl_ftpccc ftp_ccc;   /* FTP CCC options */
   bool no_signal;        /* do not use any signal/alarm handler */
+  bool global_dns_cache; /* subject for future removal */
   bool tcp_nodelay;      /* whether to enable TCP_NODELAY or not */
   bool ignorecl;         /* ignore content length */
   bool ftp_skip_ip;      /* skip the IP address the FTP server passes on to