firewall-lib.pl: Use an ipset set if a service group contains more than

firewall-lib.pl: Use an ipset set if a service group contains more than
15 ports.

The iptables multiport only supports up to 15 ports (ranges costs more).
To avoid this kind of limitation, now an ipset set will be used which
could handle up to 65k ports at once.

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