sys-fw: load default config path if non supplied

By default load the system fw config file from
/etc/opensnitchd/system-fw.json.

There're these options to specify the file to load:

 - via cli option with -fw-config-file
 - writing it in the default-config.json file:
   "FwOptions": { "ConfigPath": "..." }

If both options are empty, then the default one is used.

FIXME:
 When the cli option is used to load the fw configuration, and the main
preferences are saved, the fw is reloaded but the path to the fw config
is lost.

daemon/firewall/rules.go

@@ -37,15 +37,19 @@ type Firewall interface {
 }
 
 var (
-	fw       Firewall
-	queueNum = 0
+	fw            Firewall
+	queueNum      = 0
+	DefaultConfig = "/etc/opensnitchd/system-fw.json"
 )
 
 // Init initializes the firewall and loads firewall rules.
 // We'll try to use the firewall configured in the configuration (iptables/nftables).
 // If iptables is not installed, we can add nftables rules directly to the kernel,
 // without relying on any binaries.
 func Init(fwType, configPath, monitorInterval string, qNum *int) (err error) {
+	if configPath == "" {
+		configPath = DefaultConfig
+	}
 	if fwType == iptables.Name {
 		fw, err = iptables.Fw()
 		if err != nil {

daemon/main.go

@@ -561,7 +561,7 @@ func main() {
 	repeatPktChan = repeatQueue.Packets()
 
 	fwConfigPath := fwConfigFile
-	if cfg.FwOptions.ConfigPath != "" {
+	if fwConfigPath == "" {
 		fwConfigPath = cfg.FwOptions.ConfigPath
 	}
 	log.Info("Using system fw configuration %s ...", fwConfigPath)