review request: sendfile kqueue notification

Hi,

I'd like to start committing this to FreeBSD-HEAD:

http://people.freebsd.org/~adrian/netflix/20131211-sendfile-kqueue-11.diff

It implements kqueue notifications for sendfile so users can get an
asynchronous notification that the underlying mbufs have been freed.
This allows userland users of sendfile to know that the underlying
memory / file object can be recycled or overwritten. Right now the
only way to do this is to set SF_SYNC and this causes sendfile() to
sleep until the transaction is complete and the mbufs have been freed.

I've been testing this out locally in my lab environment and it's
running flawlessly at 30gbit/sec of TCP across 32,768 active
transmitting sockets.

I'd like to start merging this into -HEAD in small pieces to make it
easier to MFC to -10.

Thanks!



-adrian