[PATCH] using arc4random for strong randomness matters.

This is my first small personal contribution.

Motivation :

- Using fail-safe, file descriptor free solution on *BSD and Darwin system

- Somehow avoiding at the moment FreeBSD as it still uses RC4 (seemingly updated to Chacha20 for FreeBSD 12.0 and eventually backported later on).

- For implementation based on Chacha* it is known to be enough fast for the purpose.

- make check passes.

Hope it is good.

Thanks in advance.