RP-L2TP

Two patches. The first has l2tpd pass the calling number (if supplied)
from the calling number AVP into l2tp_session_lns_handle_incoming_call()'s
calling number parameter (as suggested by the "TODO" comment before said
call).

The second is a patch to the sync-pppd handler which places the calling
number and the peer host name into the environment variables
L2TP_CALLING_NUM and
L2TP_PEER_HOSTNAME respectively.

Note that the Linux /usr/sbin/pppd clobbers the environment before calling
any of its scripts such as ip-up, so you need to get at the environment
before pppd gets its hands on it. Do this by having l2tpd call a script
which in turn calls pppd, e.g. in /etc/l2tp/l2tp.conf:

section sync-pppd
pppd-path "/etc/l2tp/pppd-wrapper"

then have /etc/l2tp/pppd-wrapper do something like:

#!/bin/sh
exec /usr/sbin/pppd "$@" ipparam "$L2TP_CALLING_NUM"

This will cause pppd to pass the calling number to /etc/ppp/ip-up as $6.