[Dovecot] (Single instance) attachment storage with single-dbox

[Timo Sirainen]

Just a note to myself and whoever else cares, should be added to wiki
once it has its own page about this:

With single-dbox messages can be copied with hard linking. This means
that there can be multiple files that point to the same attachment file.
The attachment is now deleted only once the mail file's link count drops
to zero, so this works fine..

..until someone goes and starts manually copying files or maybe
restoring from backups or whatever, causing the hard links to be
replaced with separate files having link count=1, even when there are
other files pointing to the same attachment file. Now deleting this one
mail will delete the attachment too early.

I can't think of any other reasonable way to handle this though, so
unless someon has some great ideas, I think the solution is to simply
add enough warnings that message store shouldn't be accessed directly.
Maybe add some import/export commands to doveadm which can be used to
add a bunch of mails to storage without doing it directly on filesystem.