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.