arc4random(3): Reduce diff with OpenBSD.
The main change was v1.57 by djm@:
Randomise the rekey interval a little. Previously, the chacha20 instance would be rekeyed every 1.6MB. This makes it happen at a random point somewhere in the 1-2MB range.
Reviewed by: csprng (markm, cem)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D36088