pcap: implement LINKTYPE_NULL

pcap: implement LINKTYPE_NULL

Implement LINKTYPE_NULL for pcap live and pcap file.

From: http://www.tcpdump.org/linktypes.html

"BSD loopback encapsulation; the link layer header is a 4-byte field,
 in host byte order, containing a PF_ value from socket.h for the
 network-layer protocol of the packet.

 Note that ``host byte order'' is the byte order of the machine on
 which the packets are captured, and the PF_ values are for the OS
 of the machine on which the packets are captured; if a live capture
 is being done, ``host byte order'' is the byte order of the machine
 capturing the packets, and the PF_ values are those of the OS of
 the machine capturing the packets, but if a ``savefile'' is being
 read, the byte order and PF_ values are not necessarily those of
 the machine reading the capture file."

Feature ticket #1445