Forbid fork() without exec()?

On #1774 (closed) we note that rand no longer supports reseed-after-fork. In a meeting, @Diziet remarked that we shouldn't be permitting fork() at all without an immediate exec().

This ticket is for:

• Documenting that you can't fork() and expect arti to keep working.
• Possibly, detecting attempts to use arti after a fork, and panicking.