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.