Comment 11 for bug 339966

This patch covers all situations I could think off:
- install of dovecot-postfix on top of already working dovecot and postfix
  - removing dovecot-postfix in this case would bring back configuration of postfix from before installation and would rename dovecot-postfix.conf, so that real dovecot.conf would be used
- removing dovecot-postfix doesn't delete user made changes, but it does remove postfix-dovecot integration (postfix's configuration is reset to the state before the package was installed)
  - on reinstall of dovecot-postfix user made changes are returned and diff against default configuration is provided to the user; integration is back again
- purging dovecot-postfix removes all dovecot related configuration from dovecot-postfix package and resets postfix's configuration to the state before the package was installed
- upgrade of dovecot-postfix