Added inet_interfaces parameter and complete support for FreeBSD

Previously postfix was listening on all interfaces by default, but things like
relaying mail were limited using "mydestination" and "mynetworks" options. Also,
the postfix::packetfilter class defaulted to blocking all inbound access to port
25 (SMTP). Increased security by making postfix to only bind to loopback
interfaces by default, and added "inet_interfaces" parameter which can be used
to override this behavior. Also added a new FreeBSD-specific subclass,
postfix::config::freebsd to handle FreeBSD-specific postfix configuration.

Signed-off-by: Samuli Seppänen <samuli@openvpn.net>