Added syntax and implementation for BACKUP STAGE's

Part of MDEV-5336 Implement LOCK FOR BACKUP

- Changed check of Global_only_lock to also include BACKUP lock.
- We store latest MDL_BACKUP_DDL lock in thd->mdl_backup_ticket to be able
  to downgrade lock during copy_data_between_tables()

mysql-test/main/backup_priv.test

@@ -0,0 +1,52 @@
+--source include/have_innodb.inc
+--source include/not_embedded.inc
+--source include/have_metadata_lock_info.inc
+
+--echo #
+--echo # Test privileges for BACKUP STAGES
+--echo #
+
+set sql_mode="";
+
+GRANT RELOAD ON *.* TO user1@localhost;
+GRANT CREATE, DROP ON *.* TO user2@localhost;
+
+--connect(con1, localhost, user1)
+BACKUP STAGE START;
+BACKUP STAGE END;
+--echo # change_user must release backup lock
+BACKUP STAGE START;
+BACKUP STAGE FLUSH;
+SELECT lock_mode FROM information_schema.metadata_lock_info WHERE lock_type='Backup lock';
+change_user user2;
+SELECT lock_mode FROM information_schema.metadata_lock_info WHERE lock_type='Backup lock';
+--disconnect con1
+--source include/wait_until_disconnected.inc
+--connection default
+
+--echo # A very low privileged user (-> con4) cannot acquire the backup lock
+
+--connect(con1, localhost, user2)
+--error ER_SPECIFIC_ACCESS_DENIED_ERROR
+BACKUP STAGE START;
+--error ER_SPECIFIC_ACCESS_DENIED_ERROR
+BACKUP STAGE FLUSH;
+--error ER_SPECIFIC_ACCESS_DENIED_ERROR
+BACKUP STAGE BLOCK_DDL;
+--error ER_SPECIFIC_ACCESS_DENIED_ERROR
+BACKUP STAGE BLOCK_COMMIT;
+--error ER_SPECIFIC_ACCESS_DENIED_ERROR
+BACKUP STAGE END;
+--disconnect con1
+--source include/wait_until_disconnected.inc
+
+--connection default
+DROP USER user1@localhost, user2@localhost;
+
+--echo #
+--echo # Test using BACKUP STAGES in a SP
+--echo #
+
+--error ER_SP_BADSTATEMENT
+create procedure foo42()
+  BACKUP STAGE START;