MXS-2784: Use SQL to fetch default character set

The Connector-C was changed to always return only the client's charset,
not the actual charset that the connection ends up using. To cope with
this, the code has to use SQL to join the default character set name to
the default collation for it which can be used to extract the numeric ID
of the charset.

server/modules/authenticator/MySQLAuth/dbusers.cc

@@ -701,9 +701,7 @@ static bool check_server_permissions(SERVICE* service,
     }
 
     /** Copy the server charset */
-    MY_CHARSET_INFO cs_info;
-    mysql_get_character_set_info(mysql, &cs_info);
-    server->charset = cs_info.number;
+    server->charset = mxs_mysql_get_character_set(mysql);
 
     if (server->version_string[0] == 0)
     {