global_init: move the IPv6 works status bool to multi handle

Previously it was stored in a global state which contributed to
curl_global_init's thread unsafety. This boolean is now instead figured
out in curl_multi_init() and stored in the multi handle. Less effective,
but thread safe.

Closes #4851

lib/easy.c

@@ -185,8 +185,6 @@ static CURLcode global_init(long flags, bool memoryfuncs)
     goto fail;
   }
 
-  (void)Curl_ipv6works();
-
 #if defined(USE_SSH)
   if(Curl_ssh_init()) {
     goto fail;