Add a sample application using the C library.
My main goal is to make writing selftests easier but until
I have some of those ready I think it's useful to show off
the functionality and let people poke and tinker.

Sample outputs - dump:

$ ./netdev
Select ifc ($ifindex; or 0 = dump; or -2 ntf check): 0
      lo[1]	0:
  enp1s0[2]	23: basic redirect rx-sg

Notifications (watching veth pair getting added and deleted):

$ ./netdev
Select ifc ($ifindex; or 0 = dump; or -2 ntf check): -2
[53]	0: (ntf: dev-add-ntf)
[54]	0: (ntf: dev-add-ntf)
[54]	23: basic redirect rx-sg (ntf: dev-change-ntf)
[53]	23: basic redirect rx-sg (ntf: dev-change-ntf)
[53]	23: basic redirect rx-sg (ntf: dev-del-ntf)
[54]	23: basic redirect rx-sg (ntf: dev-del-ntf)
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>