Revert the patch for MDEV-18917, which removed this functionality.
This restores that mariabackup --prepare recovers the transactional
binlog position from the redo log, and writes it to the file
xtrabackup_binlog_pos_innodb.

This position is updated only on every InnoDB commit. This means that
if the last event in the binlog at the time of backup is a DDL or
non-transactional update, the recovered position from --no-lock will
be behind the state of the backup.
Signed-off-by: Kristian Nielsen <knielsen@knielsen-hq.org>