all: generalize vpn client info

VPN clients often have empty MAC address, because they come from virtual
network interface. However, there's other setup/devices also create
virtual interface, but is not VPN.

Changing source of those clients to empty to prevent confustion in
clients list command output.

internal/clientinfo/virtual_iface.go

@@ -0,0 +1,43 @@
+package clientinfo
+
+import (
+	"sync"
+)
+
+// virtualNetworkIface is the manager for clients from virtual network interface.
+type virtualNetworkIface struct {
+	ip2name sync.Map // ip  => name
+	mac     sync.Map // ip  => mac
+}
+
+// LookupHostnameByIP returns hostname of the given VPN client ip.
+func (v *virtualNetworkIface) LookupHostnameByIP(ip string) string {
+	val, ok := v.ip2name.Load(ip)
+	if !ok {
+		return ""
+	}
+	return val.(string)
+}
+
+// LookupHostnameByMac always returns empty string.
+func (v *virtualNetworkIface) LookupHostnameByMac(mac string) string {
+	return ""
+}
+
+// String returns the string representation of virtualNetworkIface struct.
+func (v *virtualNetworkIface) String() string {
+	return ""
+}
+
+// List lists all known VPN clients IP.
+func (v *virtualNetworkIface) List() []string {
+	if v == nil {
+		return nil
+	}
+	var ips []string
+	v.mac.Range(func(key, value any) bool {
+		ips = append(ips, key.(string))
+		return true
+	})
+	return ips
+}