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

internal/provider/utils/wireguard.go

@@ -9,11 +9,12 @@ import (
 )
 
 func BuildWireguardSettings(connection models.Connection,
-	userSettings settings.Wireguard) (settings wireguard.Settings) {
+	userSettings settings.Wireguard, ipv6Supported bool) (settings wireguard.Settings) {
 	settings.PrivateKey = *userSettings.PrivateKey
 	settings.PublicKey = connection.PubKey
 	settings.PreSharedKey = *userSettings.PreSharedKey
 	settings.InterfaceName = userSettings.Interface
+	settings.IPv6 = &ipv6Supported
 
 	const rulePriority = 101 // 100 is to receive external connections
 	settings.RulePriority = rulePriority