Add librss, a simple wrapper around RSS APIs so applications can begin auto-tuning.

I've used this in a handful of RSS test applications. It is just some
very simple functions to fetch the RSS configuration, query the per-bucket
CPU set, and mark sockets as local to an RSS bucket. It should be sufficient
for both thread-based and process-based workloads.

(Yes, I wrote a manpage.)

This is based on some early RSS API and wrapper API work I did whilst
I was at Netflix. Thanks to Netflix for the very original work that
spawned this; thanks to Peter Grehan for his feedback about RSS APIs
and thanks to Jack Vogel and Navdeep Parhar for the NIC-facing side of the
APIs. These fed into the simple userland API I wrote up here.

Reviewed by: gallatin