Fix link-type for nlmon.

Before patch:
  tcpdump: listening on nlmon0, link-type LINUX_SLL (Linux cooked), capture size 65535 bytes

After patch:
  tcpdump: listening on nlmon0, link-type 253, capture size 65535 bytes
Add names for DLT_NETLINK

Before patch:
  tcpdump: listening on nlmon0, link-type 253, capture size 65535 bytes

After patch:
  tcpdump: listening on nlmon0, link-type NETLINK (Linux netlink), capture size 65535 bytes
Merge pull request #334 from darkjames/master-netlink-fixes

Fixes for netlink