cmd/cli: add support for no default route systems

Currently, ctrld requires the default route interface existed to be
functional correctly.

However, on systems where default route is non existed, or point to a
virtual interface (like ipsec based VPN), the fact that the OS is using
this interface as default gateway and doesn't actually send things to
127.0.0.1 is not ctrld's problem.

In this case, ctrld should just start normally, without worrying about
the no default route interface problem.

cmd/cli/cli.go

@@ -781,7 +781,13 @@ func defaultIfaceName() string {
 		if oi := osinfo.New(); strings.Contains(oi.String(), "Microsoft") {
 			return "lo"
 		}
-		mainLog.Load().Fatal().Err(err).Msg("failed to get default route interface")
+		// On linux, it could be either resolvconf or systemd which is managing DNS settings,
+		// so the interface name does not matter if there's no default route interface.
+		if runtime.GOOS == "linux" {
+			return "lo"
+		}
+		mainLog.Load().Debug().Err(err).Msg("no default route interface found")
+		return ""
 	}
 	return dri
 }