rand/: Secure `rand' generator against fork problems.

rand/: Secure `rand' generator against fork problems.

This is fiddlier than it really ought to be.

  * Make the `i' and `irot' members be `unsigned short' to make space
    for a new member.  These members have well-constrained ranges, so
    this is safe.

  * Insert a new `gen' member to keep track of the pool's `generation
    number'.  Arrange that the global generator's generation number is
    initially zero.

  * Invent a new system-specific function `rand_generation' which
    returns a nonzero `generation number', which changes across forks
    and such things.

  * Have the output functions `rand_get' and `rand_getgood' check the
    generation number and force a `rand_gate' if it changes.

  * Arrange for `rand_gate' and `rand_stretch' to feed the generation
    number into the hashing, so that generators with different
    generations behave computationally independently.