Add new boolean option $mail_check_stats (default off) and
$mail_check_stats_interval.  The first turns extended buffy on.  The
second sets the amount of time in between extended buffy checks
(defaulting to 60 seconds).

Remove the option $sidebar_refresh_time.

Change mutt_buffy_check() to only notify the sidebar to redraw if a
mailbox buffy value changes.

Remove the #ifdefs around the extended buffy functions.  The next
patch will merge these functions with the basic functions and pass a
parameter instead.

Imap is a special case, because it sends out the status in one batch.
Change this to perform the comparisons inside cmd_parse_status() and
flag the sidebar there.  It was previously directly assigning the
status counters (unsigned int) to the buffy->new (short).  Change this
to assign 1/0.