firewall: rules.pl: Fix rules with other NAT port.
Firewall: Move some functions from rules.pl to firewall-lib.pl
Firewall: DNAT - Show right DNAT interface in ruletable

Now:
When using a hostgroup as source there are all corresponding DNAT
interfaces shown in ruletable depending on the entries in the group.

When in DNAT area "-automatic" is selected, the DNAT interfaces are
shown as IP-Addresses, else they are shown as "ORANGE","GREEN","BLUE"...

BUGFIX: When there is a MAC address used in a sourcegroup, the rules could not be set. Now MAC addresses get allways the public interface as DNAT
Firewall: Rename defaultNetworks to netsettings
Firewall: Allow DNAT with target firewall
firewall: rules.pl: Allow REDIRECT rules.
firewall: rules.pl: Catch invalid configurations.
firewall: rules.pl: Honour time constraints for NAT rules as well.