guardian.cgi: Add function to generate the guardian.ignore file.

guardian.cgi: Add function to generate the guardian.ignore file.

This function is responsible for collecting all required data,
like the green, blue, orange (if the interfaces are available),
red, gateway and used DNS server IP-addresses.

It will add als these addresses and the configured and enabled
user-defined ignored addresses/networks to the ignore file of
guardian to prevent from blocking any of them.

Note:

The IPFire and RED inteface related addresses also will be added
to the ignore file, even if there is no user-defined entry in the
list.

Signed-off-by: Stefan Schantl <stefan.schantl@ipfire.org>