Deprecate and ignore innodb_undo_logs, remove innodb_rollback_segments

The option innodb_rollback_segments was deprecated already in MariaDB Server 10.0. Its misleadingly named replacement innodb_undo_logs is of very limited use. It makes sense to always create and use the maximum number of rollback segments.

Let us remove the deprecated parameter innodb_rollback_segments and deprecate&ignore the parameter innodb_undo_logs (to be removed in a later major release).

This work involves some cleanup of InnoDB startup. Similar to other write operations, DROP TABLE will no longer be allowed if innodb_force_recovery is set to a value larger than 3.