all: watch lease files if send client info enabled

So users who run ctrld in Linux can still see clients info, even though
it's not an router platform that ctrld supports.

client_info.go

@@ -9,3 +9,11 @@ type ClientInfo struct {
 	IP       string
 	Hostname string
 }
+
+// LeaseFileFormat specifies the format of DHCP lease file.
+type LeaseFileFormat string
+
+const (
+	Dnsmasq  LeaseFileFormat = "dnsmasq"
+	IscDhcpd                 = "isc-dhcpd"
+)