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/perfectprivacy/openvpnconf.go

@@ -8,7 +8,7 @@ import (
 )
 
 func (p *Provider) OpenVPNConfig(connection models.Connection,
-	settings settings.OpenVPN) (lines []string) {
+	settings settings.OpenVPN, ipv6Supported bool) (lines []string) {
 	//nolint:gomnd
 	providerSettings := utils.OpenVPNProviderSettings{
 		AuthUserPass: true,
@@ -33,5 +33,5 @@ func (p *Provider) OpenVPNConfig(connection models.Connection,
 			`pull-filter ignore "redirect-gateway def1 ipv6"`,
 		},
 	}
-	return utils.OpenVPNConfig(providerSettings, connection, settings)
+	return utils.OpenVPNConfig(providerSettings, connection, settings, ipv6Supported)
 }