Drop thread_fast_rng during postfork; improve thread_fast_rng fork-safety

If you fork with a thread_fast_rng, we need to invalidate the thread_fast_rng so it will get reconstructed in the child process.

We should also tweak thread_fast_rng so that it is always misuse-resistant across fork()s in the same way. Right now it has 3 behaviors if you try to share it across a fork:

• it works seamlessly if you have zero-on-fork support
• it crashes if you have drop-on-fork support
• it becomes an insecure prng if you have neither.