feat: add --rfc1918 flag for explicit LAN client support

Make RFC1918 listener spawning opt-in via --rfc1918 flag instead of automatic behavior.
This allows users to explicitly control when ctrld listens on private network addresses
to receive DNS queries from LAN clients, improving security and configurability.

Refactor network interface detection to better distinguish between physical and virtual
interfaces, ensuring only real hardware interfaces are used for RFC1918 address binding.

cmd/cli/commands.go

@@ -189,6 +189,7 @@ func initRunCmd() *cobra.Command {
 	runCmd.Flags().StringVarP(&iface, "iface", "", "", `Update DNS setting for iface, "auto" means the default interface gateway`)
 	_ = runCmd.Flags().MarkHidden("iface")
 	runCmd.Flags().StringVarP(&cdUpstreamProto, "proto", "", ctrld.ResolverTypeDOH, `Control D upstream type, either "doh" or "doh3"`)
+	runCmd.Flags().BoolVarP(&rfc1918, "rfc1918", "", false, "Listen on RFC1918 addresses when 127.0.0.1 is the only listener")
 
 	runCmd.FParseErrWhitelist = cobra.FParseErrWhitelist{UnknownFlags: true}
 	rootCmd.AddCommand(runCmd)
@@ -531,6 +532,7 @@ NOTE: running "ctrld start" without any arguments will start already installed c
 	startCmd.Flags().BoolVarP(&skipSelfChecks, "skip_self_checks", "", false, `Skip self checks after installing ctrld service`)
 	startCmd.Flags().BoolVarP(&startOnly, "start_only", "", false, "Do not install new service")
 	_ = startCmd.Flags().MarkHidden("start_only")
+	startCmd.Flags().BoolVarP(&rfc1918, "rfc1918", "", false, "Listen on RFC1918 addresses when 127.0.0.1 is the only listener")
 
 	routerCmd := &cobra.Command{
 		Use: "setup",