add libottery RNG implementation, replace the portable version from OpenSSH

The OpenSSH portable arc4random implementation relies on OpenSSL to seed
it, which creates a circular dependency here.

libottery, while alpha software (just like this is), portably does a
more reasonable job than any wheel I could have reinvented for
performing portable crypto seeding. It implements the same ChaCha stream
cipher as OpenBSD's somewhat misnamed arc4random implementation.

Be sure to read and understand https://github.com/nmathewson/libottery
along with associated warnings.