Add a value to old_mode variable to disable the logic introduced in MDEV-16329, e.g. NO_ONLINE_COPY_ALTER or NO_NOLOCK_COPY_ALTER.

Note: Just NO_ONLINE_ALTER doesn't work, as it's ambiguous due to the old ALTER ONLINE syntax.

The value should not be set by default.

If it is set,

• ALTER TABLE without explicit ALGORITHM=COPY, LOCK=NONE should fall back to the old logic;
• ALTER TABLE with explicit ALGORITHM=COPY, LOCK=NONE can be either allowed or forbidden, whichever is easier to implement.