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

Dockerfile

@@ -89,7 +89,6 @@ ENV VPNSP=pia \
     DNS_KEEP_NAMESERVER=off \
     # Firewall
     FIREWALL=on \
-    EXTRA_SUBNETS= \
     FIREWALL_VPN_INPUT_PORTS= \
     FIREWALL_INPUT_PORTS= \
     FIREWALL_DEBUG=off \