ids-functions.pl: Add function to get the available network zones

ids-functions.pl: Add function to get the available network zones

The get_available_network_zones() function uses the /var/ipfire/ethernet/settings
file and translates the configured mode into an array, which contains the names
of the configured network zones.

The array will be returned and easily can be used to loop over this list of
available network zones and perform any kind of actions in other scripts.

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