Feat: OPENVPN_INTERFACE defaulting to tun0

- Fix: custom config with custom network interface name for firewall
- Keep VPN tunnel interface in firewall state
- Vul fix: only allow traffic through vpn interface when needed
- Adapt code to adapt to network interface name
- Remove outdated TUN and TAP constants

internal/firewall/firewall.go

@@ -40,6 +40,7 @@ type Config struct { //nolint:maligned
 	// State
 	enabled           bool
 	vpnConnection     models.Connection
+	vpnIntf           string
 	outboundSubnets   []net.IPNet
 	allowedInputPorts map[uint16]string // port to interface mapping
 	stateMutex        sync.Mutex