iproute2: add l2spec_type param to l2tp add session

iproute2: add l2spec_type param to l2tp add session

When unmanaged L2TP sessions are created using "ip l2tp add session",
there is no option to allow the session's Layer2SpecificHeader type to
be selected - the kernel's default setting is always used. For
interopability with some vendor equipment, it might be necessary to
use a different setting. So add a new l2spec_type parameter to the "ip
l2tp add session" parameter list, allowing operators to set a specific
Layer2SpecificHeader type. The kernel already exposes the setting as a
netlink attribute so it is straightforward to add support for it in
iproute2.

This change allows unmanaged L2TP sessions to be configured between
Linux and some Cisco equipment by specifying "l2spec_type none" in "ip
l2tp add session" command parameters.

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