curl_global_init: Check if IPv6 works

- Curl_ipv6works() is not thread-safe until after the first call, so
call it once during global init to avoid a possible race condition.

Bug: https://github.com/curl/curl/issues/915
PR: https://github.com/curl/curl/pull/918

lib/easy.c

@@ -271,6 +271,8 @@ static CURLcode global_init(long flags, bool memoryfuncs)
     return CURLE_FAILED_INIT;
   }
 
+  (void)Curl_ipv6works();
+
 #if defined(USE_LIBSSH2) && defined(HAVE_LIBSSH2_INIT)
   if(libssh2_init(0)) {
     DEBUGF(fprintf(stderr, "Error: libssh2_init failed\n"));