MDEV-6871 Multi-value insert on MyISAM table that makes slaves crash (when using --skip-external-locking=0)
Problem was that repair() did lock and unlock tables, which leaved already locked tables in wrong state

include/my_check_opt.h:
  Added option T_NO_LOCKS to disable locking during repair()
  Fixed duplicated bit T_NO_CREATE_RENAME_LSN
mysql-test/suite/rpl/r/myisam_external_lock.result:
  Test case for MDEV-6871
mysql-test/suite/rpl/t/myisam_external_lock-slave.opt:
  Test case for MDEV-6871
mysql-test/suite/rpl/t/myisam_external_lock.test:
  Test case for MDEV-6871
storage/maria/ha_maria.cc:
  Don't lock tables during enable_indexes()
  Removed some calls to current_thd
storage/myisam/ha_myisam.cc:
  Don't lock tables during enable_indexes()
  Removed some calls to current_thd