[L2TP]: PPP over L2TP driver core

This driver handles only L2TP data frames; control frames are handled
by a userspace application. It implements L2TP using the PPPoX socket
family. There is a PPPoX socket for each L2TP session in an L2TP
tunnel.  PPP data within each session is passed through the kernel's
PPP subsystem via this driver. Kernel parameters of each socket can be
read or modified using ioctl() or [gs]etsockopt() calls.

Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>