mariabackup should update binlog position in InnoDB during prepare

Mariabackup currently stores the backup's binary log position in the file xtrabackup_binlog_info.

https://mariadb.com/kb/en/files-created-by-mariabackup/#xtrabackup_binlog_info

InnoDB also stores the data's corresponding binary log position in its global tablespace.

It seems as though mariabackup does not currently update the binary log position stored in InnoDB during the prepare. This has a tendency to confuse users, since they see mismatching binary log positions.

For example, they might see something like this in the prepare output written by mariabackup:

2020-01-31 1:58:55 0 \[Note\] InnoDB: Last binlog file '/binlogs/mysql\-bin.014960', position 54640630

\[00\] 2020-01-31 01:58:56 Last binlog file /binlogs/mysql\-bin.014960, position 54640630

\[00\] 2020-01-31 01:58:57 completed OK!

And they might see a mismatched binary log position in xtrabackup_binlog_info:

$ cat /mariabackup/base/2020-01-31_00-06-18/xtrabackup_binlog\_info

mysql\-bin.014961 449

To prevent confusion, when mariabackup prepares a backup, it should probably parse the binary log position from xtrabackup_binlog_info, and then use that position to update the binary log position in InnoDB.