hotfix(protonvpn): port forward only option

- Allow to use with Protonvpn (not just PIA)
- Update code comments
- Check server supports port forwarding when invoking port forward code

internal/vpn/tunnelup.go

@@ -11,7 +11,7 @@ type tunnelUpData struct {
 	// Port forwarding
 	vpnIntf        string
 	serverName     string // used for PIA
-	canPortForward bool   // used for PIA
+	canPortForward bool   // used for PIA and ProtonVPN
 	username       string // used for PIA
 	password       string // used for PIA
 	portForwarder  PortForwarder