Add netmap support to libpcap. Tcpdump and other native pcap application can now
run directly on netmap ports using netmap:foo or valeXX:YY device names.
Modifications to existing code are small and trivial, the netmap-specific
code is all in a new file.

Please be aware that in netmap mode the physical interface is disconnected from
the host stack, so libpcap will steal the traffic not just make a copy.

For the full version of the code (including linux and autotools support) see
https://code.google.com/p/netmap-libpcap/

MFC after: 3 days