mailbox: add open-hook support aka compressed

taken from debian/features/compressed

URL: http://www.spinnaker.de/mutt/compressed/

"Stability disclaimer: You should not use this patch for incoming mail
folders, but only for archive folders, because there is no mechanism yet
to synchronize or merge the (temporary) uncompressed folder (which is
created while you read the messages in this folder) and the compressed
folder where new incoming mail would be appended. This implies, that if
you read a compressed folder with Mutt while some other process (either
another Mutt process or a mail delivery agent like procmail) writes to
this folder, the new mail will be lost. So be careful!"

No problems were found when working with archived compressed mailboxes.

Can be used to see the treaded discussions when the user is not
subscribed to a mailinglist and is not CCed on the replies.

Original-author: Alain Penders <Alain@Finale-Dev.com>
Original-author: Roland Rosenfeld <roland@spinnaker.de>
Signed-off-by: David Sterba <dsterba@suse.cz>