Backup doesn't respect the users=none setting at all

I was playing with MDL-10721 when I've detected that the backup_users = none setting isn't respected by a lot of parts of backup.

Correctly, the <USERS> section is missing in the backup file, but:

• User grades are exported.
• Activities user info are exported (responses, modules...)
• Header info of activities show USERINFO = yes, while it should be no!

This causes backup to export a lot of unneeded data, spending a lot of extra time and making difficult to compare scheduled backups and manual backups, so I'll try this first.

Ciao