multi: probe for IPv6 functionality in multi_init()

In some legacy systems IPv6 might dynamically work/not work and thus
curl needs to check/probe to see if it should indeed be used.

This change moves the probe that checks for working IPv6 to the multi
handle setup function instead of delaying it to when the first name
resolve is performed. This avoids a later tricky error path if the
socket cannot be created due to OOM.

Closes #20383

lib/multi.c

@@ -300,6 +300,9 @@ struct Curl_multi *Curl_multi_handle(uint32_t xfer_table_size,
   }
 #endif
 
+  if(Curl_probeipv6(multi))
+    goto error;
+
   return multi;
 
 error: