netmap: initial import of netmap core

Vanilla sources; not hooked up to the build.

Taken-from: https://code.google.com/p/netmap/ (3b385fe7854d)
netmap: lock(9), k*() foo, no ifdefs

The ifdef glue is a bit hard to see through.  Remove it now to
bring it back later with appropriate ifdefs for DragonFly.
netmap: revamped documentation