Add a check_stats parameter to the mbox, maildir, and mh buffy
functions.  Use that parameter to determine whether to also count
total, new, and flagged messages.

This makes the functions a bit more complicated, but improves
efficiency (for maildir and mh).

Also includes the following cleanup/fixes:
* Move the orig-value counter reset to the beginnining of the loop,
  (before tmp->new is set to 0).

* Change trashed maildir messages to not be counted in msg_count

* Remove an incorrect setting of mailbox->new based on msg_count in
  maildir.  (I missed this one for 1f840760e6e0)

* Change mbox to use the context->mtime for stats_last_checked,
  removing a race condition.

* Fix mh to actually count the messages in order to generate msg_count.
  mh_sequences only covers the range of messages with some sort of
  flag.