MDEV-26: Global transaction ID.

Replace CHANGE MASTER TO ... master_gtid_pos='xxx' with a new system
variable @@global.gtid_pos.

This is more logical; @@gtid_pos is global, not per-master, and it is not
affected by RESET SLAVE.

Also rename master_gtid_pos=AUTO to master_use_gtid=1, which again is more
logical.

mysql-test/include/mtr_check.sql

@@ -31,6 +31,7 @@ BEGIN
      AND variable_name not like "Last_IO_Err*"
      AND variable_name != 'INNODB_IBUF_MAX_SIZE'
      AND variable_name != 'INNODB_USE_NATIVE_AIO'
+     AND variable_name != 'GTID_POS'
    ORDER BY variable_name;
 
   -- Dump all databases, there should be none