remote_allow_ranges: allow inside CIDR specific remote_allow_lists (#540)

This allows you to configure remote allow lists specific to different
subnets of the inside CIDR. Example:

    remote_allow_ranges:
      10.42.42.0/24:
        192.168.0.0/16: true

This would only allow hosts with a VPN IP in the 10.42.42.0/24 range to
have private IPs (and thus don't connect over public IPs).

The PR also refactors AllowList into RemoteAllowList and LocalAllowList to make it clearer which methods are allowed on which allow list.

handshake.go

@@ -6,7 +6,8 @@ const (
 )
 
 func HandleIncomingHandshake(f *Interface, addr *udpAddr, packet []byte, h *Header, hostinfo *HostInfo) {
-	if !f.lightHouse.remoteAllowList.Allow(addr.IP) {
+	// First remote allow list check before we know the vpnIp
+	if !f.lightHouse.remoteAllowList.AllowUnknownVpnIp(addr.IP) {
 		f.l.WithField("udpAddr", addr).Debug("lighthouse.remote_allow_list denied incoming handshake")
 		return
 	}