Support IMAP prefix

In one mailserver I have for legacy reasons an IMAP prefix set in dovecot. In was migrated from courier.

The prefix I have is 'INBOX.'

While looking into the 500-clean_mailboxes.inc.php code I noticed that it tries to empty folders with and without a prefix. But not for all, I'm assuming that's just an oversight.

But since I'm already patching the /sieve_filter.master template to include a prefix I though it might be a good idea to have this as an option.

I'm preparing a branch for this. Let me know what you think..