Add unread and total message count format strings to $folder_format.
%n will show the unread message count in the mailbox. %m will show
total message count. Except for %n with IMAP, these both require
$mail_check_stats to be set, which puts these counts inside BUFFY.
Since the imap_mboxcache is never fresher than the value in BUFFY,
remove the special imap_mailbox_state() call. Instead, just update
from the current Context for all mailboxes.
Remove the logic that overrode the %N format string to show unread
count for IMAP mailboxes. If they want to see unread count, they will
have to change the $folder_format.
Although it doesn't look possible to reuse browser_state.entry slots,
change the OP_DELETE_MAILBOX to memset(0) the deleted slot. Then, we
can change to logic in add_folder() so it doesn't need to zero-out
unset values.