tunnel: Userspace implementation of tunnel manipulation.

tunnel: Userspace implementation of tunnel manipulation.

The kernel tunneling code currently needs to handle a large number
of operations when tunnel packets are encapsulated and
decapsulated.  Some examples of this are: finding the correct
tunnel port on receive, TTL and ToS inheritance, ECN handling, etc.
All of these can be done on a per-flow basis in userspace now that
we have both the inner and outer header information, which allows
us to both simplify the kernel and take advantage of userspace's
information.  Once tunnel packets are redirected into this code,
the redundant pieces can be removed from other places.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Signed-off-by: Ethan Jackson <ethan@nicira.com>