Extend fwall-drv interface and call functions via interface

Extend fwall-drv interface and call functions via interface

I am moving some of the eb/iptables related functions into the interface
of the firewall driver and am making them only accessible via the driver's
interface. Otherwise exsiting code is adapted where needed. I am adding one
new function to the interface that checks whether the 'basic' rules can be
applied,  which will then be used by a subsequent patch.