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- Remove outdated TUN and TAP constants

Dockerfile

@@ -76,6 +76,7 @@ ENV VPNSP=pia \
     OPENVPN_TARGET_IP= \
     OPENVPN_IPV6=off \
     OPENVPN_CUSTOM_CONFIG= \
+    OPENVPN_INTERFACE=tun0 \
     TZ= \
     PUID= \
     PGID= \