Add L2TP support.

Add L2TP support.

Patch from James Chapman.

This patch adds support for L2TP. It allows pppd to interface with the
pppol2tp driver in the Linux kernel. All data packets are handled by
the Linux kernel in order that the datapath be as efficient as
possible, while a userspace daemon implements the L2TP control
protocol, handling tunnel/session setup and teardown. The
implementation uses the PPPoX infrastructure; the architecture is
similar to PPPoE/PPPoATM in that a userspace daemon spawns a pppd
process per PPP session and uses a protocol-specific plugin to connect
pppd with the kernel.

The pppol2tp Linux kernel driver was integrated in the Linux kernel
from 2.6.23. For earlier kernels, an out of tree driver is available
from the pppol2tp-kmod package on the OpenL2TP project site at
http://sourceforge.net/projects/openl2tp.

Signed-off-by: James Chapman <jchapman@katalix.com>