moved dovecot-lmtpd from install/update to server-plugin to prevented dovecot-errors when using dovecot without dovecot-lmtpd

Use dovecot or maildrop in default-cases
Switching to LMTP and back included in server-postfix-plugin
validator added to disable switching uid-mapping when already created mailusers or in multiserver-environment

Fixed: FS#3032 - dovecot: master: Warning: /var/www/clients/client20/web36/log is no longer mounted. If this is inten

additionsl configuration-changes when update -> use lmtp instead of dovecot directly

Implemented FS#2593 - PHP Warning: chmod(): No such file or directory in /root/ispconfig3_install/install/dist/lib/debian60.lib.php

- I had to remove the fail2ban caonfig for now as its incomplete and adds rules for dovecot on courier systems.
It is planned to add a complete fail2ban configuration in one of the next releases.

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