unread flag and maildir syncing

[David Bremner]

1/2 is just a fixup for a typo that I made (and Tomi missed ;))
2/2 is to help decide the desired behaviour without 'unread' in new.tags

The current behaviour is that maildir flags win:

| unread in new.tags | ,S on file name | unread tag in db |
| Y                  | Y               | N                |
| N                  | Y               | N                |
| Y                  | N               | Y                |
| N                  | N               | Y                |

This means in particular that files in new/ have unread whether or not
the user specifies that in new.tags.  That might be the right
behaviour (it's certainly easier to impliment). I just want to confirm
either way before I update notmuch-new (and potentially drop the
"unread is not mandatory in new" test)

d