Rendering deltas with rates

Given two snapshots, and the interval between them, we can compute the rate that derive counters are increasing by.  This patch adds new functions for rendering two snapshots that includes rates for any derives.  (The rate portion is left blank for gauges.)

The periodic interface has also been updated to keep track of the previous snapshot, along with the current one, to make it easy for background threads to calculate rates.
Yeah it's because those `**`s around the function name don't take up space when they're rendered.  So you indent four spaces less that it looks like you should.  If you run `man docs/bws_snapshot_render.3` you can verify that it looks right after rendering.