Added deprecation notice to SIEVE port configuration property.

The new port 4190 relates to Debian Squeeze (also UCS 3.0) and other more recent Linux distributions and IMAP implementations (Dovecot 2 / Pigeonhole SIEVE).

The old port (2000/tcp) is now assigned to some Cisco SCCP protocol by the IANA.

Our default setting at points to the deprecated port number. I'm sure many installations still use 2000/tcp but at least we should put some note at the configuration file. It may be a good default to use the system default port defined at /etc/services.