netsniff-ng: Add netlink dissector

Add an initial implementation of a dissector to work on netlink messages
as received from an nlmon device.

Use can use it as follows to monitor netlink traffic to/from the kernel:

  modprobe nlmon
  ip link add type nlmon
  ip link set nlmon0 up

  netsniff-ng -i nlmon0

  ip link set nlmon 0 down
  ip link del dev nlmon0
  rmmod nlmon

Fixes: #89
Suggested-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>