Routing improvements (#268)

- Fixes #82 
- Remove `EXTRA_SUBNETS`
- Remove no longer needed iptables rules
- Reduce routing interface arity
- Routing setup is done in main.go instead of in the firewall
- Routing setup gets reverted at shutdown

internal/params/params.go

@@ -41,7 +41,6 @@ type Reader interface {
 
 	// Firewall getters
 	GetFirewall() (enabled bool, err error)
-	GetExtraSubnets() (extraSubnets []net.IPNet, err error)
 	GetVPNInputPorts() (ports []uint16, err error)
 	GetInputPorts() (ports []uint16, err error)
 	GetFirewallDebug() (debug bool, err error)