lei <q|up>: set \Recent on non-empty mbox and Maildir
Despite JMAP not supporting the equivalent of the IMAP \Recent
flag, it is useful for "lei q --augment", and "lei up" users to
be able to distinguish new results from old-but-unread messages
in an mbox or Maildir.
For mbox family messages, we'll drop the "O" status flag when
appending to mboxes, and we'll write to the "new" subdirectory
of Maildirs.
Behavior when writing to initially empty Maildirs and mboxes
remains unchanged since there's no need to distinguish between
new and old results in the initial case. Having users wait
for a rename(2) storm or complete mbox rewrite hurts UX.
With IMAP mailboxes, \Recent is already enforced by the IMAP
server and IMAP clients have no way of changing it(*)
(*) mutt uses the "Old" IMAP flag which isn't part of RFC 3501,
other MUAs may do similar things.