fix(ppp): use wildcard server name in chap/pap-secrets for pppd compatibility

internal/vpn/ppp_server.go

@@ -384,10 +384,9 @@ func (m *PPPServerManager) SyncSecretsToSystem() error {
 		if s.Disabled {
 			continue
 		}
+		// In Linux pppd, the second field is the system name of the server (e.g. "l2tpd" from xl2tpd).
+		// Setting this to "*" allows pppd to authenticate the user across L2TP, PPTP, and PPPoE daemons.
 		serviceName := "*"
-		if s.Service != "any" && s.Service != "" {
-			serviceName = s.Service
-		}
 		ipAddr := "*"
 		if s.RemoteAddress != "" {
 			ipAddr = s.RemoteAddress