rand/rand.[ch]: Add external `rand_quick' function.

rand/rand.[ch]: Add external `rand_quick' function.

This is a function which can be called frequently to top up the internal
entropy.

Internally, rename `TIMER' to `QUICK', and call the internal `quick'
function as well as the noise-source timer.  The `quick' core currently
doesn't do anything, but will act as a dispatcher to CPU-specific
entropy sources.