There are only about 4 protocols that the current ppp code handles and
it is doubtful that it will grow much more. There's no point in having
an extensive packet handler registration framework.
change ppp_decode to store the length of the decoded frame, so that
if we have a packet with a protocol we don't understand, we can send
Protocol-Reject packets. Modify ppp_cp code to add support for sending
Protocol-Reject packet.