lib: Only synchronize maildir flags for messages in maildirs

Previously, we synchronized flags to tags for any message that looked
like it had maildir flags in its file name, regardless of whether it
was in a maildir-like directory structure.  This was asymmetric with
tag-to-flag synchronization, which only applied to messages in
directories named new/ and cur/ (introduced by 95dd5fe5).

This change makes our interpretation stricter and addresses this
asymmetry by only synchronizing flags to tags for messages in
directories named new/ or cur/.  It also prepares us to treat messages
in new/ as maildir messages, even though they lack maildir flags.