Update arc4random with newer ChaCha-based hash from OpenBSD/OpenSSH

This version takes the seeding and thread safety functions from libbsd's arc4random and
combines them with OpenSSH's portable arc4random ChaCha implementation.

The stock OpenSSH arc4random seeds itself from OpenSSL's original random number generator,
which has been removed in libressl in favor of arc4random. We couldn't use this directly,
of course, because it creates a circular dependency.