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- Remove outdated TUN and TAP constants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/constants/openvpn.go

@@ -1,10 +1,5 @@
 package constants
 
-const (
-	TUN = "tun0"
-	TAP = "tap0"
-)
-
 const (
 	AES128cbc = "aes-128-cbc"
 	AES256cbc = "aes-256-cbc"