One bug fixed: Use getmicrouptime() to trigger reseeds so that we
cannot be tricked by a clock being stepped backwards.
Express parameters in natural units and with natural names.
Don't use struct timeval more than we need to.
Various stylistic and readability polishing.
Introduce arc4rand(void *ptr, u_int len, int reseed) function which
returns a stream of pseudo-random bytes, observing the automatic
reseed criteria as well as allowing forced reseeds.
Rewrite arc4random() in terms of arc4rand().
Sponsored by: DARPA & NAI Labs.
