dovecot requires either a prefix value or NULL.  If supplied an empty
string it will give an error.  This change uses NULLIF() to convert
empty strings to NULL.

Added prefetching to dovecot to halve database-queries
Added mdbox-creation for new Mailboxes in mdbox-format

Conflicts:
	install/tpl/debian6_dovecot-sql.conf.master
	install/tpl/debian_dovecot-sql.conf.master
	install/tpl/debian_postfix.conf.master
	install/tpl/fedora_dovecot-sql.conf.master
	install/tpl/fedora_postfix.conf.master
	install/tpl/gentoo_postfix.conf.master
	install/tpl/opensuse_dovecot-sql.conf.master
	install/tpl/opensuse_postfix.conf.master
	interface/web/admin/templates/server_config_mail_edit.htm
	server/cron_daily.php
	server/plugins-available/mail_plugin.inc.php
	server/plugins-available/postfix_server_plugin.inc.php

Fixed: FS#3350 - [Dovecot] Warning: mysql: Query failed, retrying: Unknown column 'filter' in 'where clause'

Cleaned and shortened to be more manageable.  There is no reason to keep all that luggage. Also use multi line sql so a human can read it.

Merged revisions 1832-1836 from stable (added dovecot support for debian squeeze and added a new distid and class for debian6 specific modifications.

Added Support for Sieve mail filtering language on dovecot servers.