Firewall now copes with backup interfaces.

Backup interfaces have all their proper routes and firewall rules set, they are just not used for Internet connections.
New variable BAKIF created to contain list of backup interfaces.
WANIF now contains all WAN interfaces (backup and primary) - use SYSWATCH_WANIF if you want just primaries.
New function PruneInterfaceTable to remove invalid interfaces from all the above tables (instead of repeating the code slightly differently for each one).