Use a triton context per session so that log_ppp_*() functions can
prepend log messages with PPP session information.

Since functions must now be called within the right context,
allocating and freeing sessions is done in the following manner:
  * Tunnels allocate new sessions within their own context and
    activate the new session's context just before answering
    the ICRQ.
  * Freeing sessions is slightly more complex. The session is
    first removed from its tunnel session list (done within the
    tunnel context), then the session itself if disconnected and
    freed (done within the session context).

The l2tp_tunnel_*() and the l2tp_session_*() functions must now be
used within tunnel and session contexts respectively.
Signed-off-by: NGuillaume Nault <g.nault@alphalink.fr>