Merge from vendor branch TNF:

Merge from vendor branch TNF:
This package is part of the Pigeonhole project (http://pigeonhole.dovecot.org).
It adds support for the Sieve language (RFC 5228) and the ManageSieve protocol
(RFC 5804) to the Dovecot Secure IMAP Server.

The Sieve language is used to specify how e-mail needs to be processed. By
writing Sieve scripts, users can customize how messages are delivered, e.g.
whether they are forwarded or stored in special folders. Unwanted messages can
be discarded or rejected, and, when the user is not available, the Sieve
interpreter can send an automated reply. Above all, the Sieve language is meant
to be simple, extensible and system independent. And, unlike most other mail
filtering script languages, it does not allow users to execute arbitrary
programs. This is particularly useful to prevent virtual users from having full
access to the mail store. The intention of the language is to make it impossible
for users to do anything more complex (and dangerous) than write simple mail
filters.

Using the ManageSieve protocol, users can upload their Sieve scripts remotely,
without needing direct filesystem access through FTP or SCP. Additionally,
aManageSieve server always makes sure that uploaded scripts are valid,
preventing compile failures at mail delivery.

This package provides Sieve support as a plugin to Dovecot's Local Delivery
Agent (LDA) and Dovecot's LMTP service. The ManageSieve protocol is provided is
an additional service, next to Dovecot's own POP3 and IMAP services.