rand/rand.c: Mix the pool key in `rand_gate' and `rand_stretch'.

Back in commit d6fab4f6ae209afd6799a2974ce2849123965170, I rearranged
the cryptography to use plain SHA256 rather than RIPEMD160-HMAC for
determining the cipher key for churning the generator state.

I also managed to remove all the points at which the key actually
influences the behaviour of the generator!

This was four years ago, and I can't remember exactly why, but my guess
is that the key was previously inserted as part of `rmd160_macinit',
which was replaced by the unheyed `HASH_INIT' function.