- Allow database level access via `LOCK TABLES` to execute statement
`BACKUP [un]LOCK <object>`
- `BACKUP UNLOCK` works only with `RELOAD` privilege.
  In case there is `LOCK TABLES` privilege without `RELOAD` privilege,
  we check if backup lock is taken before.
  If it is not we raise an error of missing `RELOAD` privilege.
- We had to remove any error/warnings from calling functions because
`thd->get_stmt_da()->m_status` will be set to error and will break
`my_ok()`.
- Added missing test coverage of `RELOAD` privilege to `main.grant.test`
Reviewer: <daniel@mariadb.org>