linktype: add netlink link/dlt type

With Linux 3.11, we have the possibility to debug local netlink traffic
[1] i.e.  the workflow looks like this:

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

Capture:
  tcpdump -i nlmon0 ...

Teardown:
  ip link set nlmon0 down
  ip link del dev nlmon0
  rmmod nlmon

For pcap interoperability, introduce a common link type for netlink
captures.