When setting the value of a message's 'can_read' property, ignore the PRIVATE message attribute (which is sometimes set in FTN netmail messages) since it's assumed all messages in the mail base are private, no special destination (to) name matching is needed here.

This only popped up recently via msglist.js because of the recent addition of checking each messages's 'can_read' property.

As reported by <Diehard> via IRC PM.