squid: Include /etc/squid/squid.conf.pre.local.

This configuration file is included _before_ the
default IPFire configuration.
postfix: Do proper configuration.

Adds LDAP support because the libs are always installed.
Disables IPv6, because IPFire 2 does not support IPv6.
The rest is cleanup stuff.
Merge branch 'next' of ssh://git.ipfire.org/pub/git/ipfire-2.x into next