feat(vpn): auto detection of IPv6 support

- `OPENVPN_IPV6` removed
- Affects OpenVPN
- Use the same mechanism for OpenVPN and Wireguard
- Check only once at program start since this is unlikely to change at runtime
- Log if IPv6 is supported
- Remove `IPv6` boolean from settings structs
- Move IPv6 detection as a method on NetLinker

Dockerfile

@@ -91,7 +91,6 @@ ENV VPN_SERVICE_PROVIDER=pia \
     OPENVPN_CIPHERS= \
     OPENVPN_AUTH= \
     OPENVPN_PROCESS_USER= \
-    OPENVPN_IPV6=off \
     OPENVPN_CUSTOM_CONFIG= \
     # Wireguard
     WIREGUARD_PRIVATE_KEY= \