arc4random(3): Reduce diff with OpenBSD.

[delphij]

Summary

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.

MFC after: 2 weeks

[delphij]

r1.56 -- Not revalent to us
r1.57 -- the main change
r1.58 -- indent fix (follow up of r1.57)

This is mostly for diff reduction; the rs_count value of 1600000 was an arbitrary value.

With the change, we would use the entropy obtained from kernel to create a "fuzz" value for the initial rs_count when doing stir, the new rs_count would now be in range of [1048576,2097151]. This makes the stir interval less predicable for a very small cost for each stir.

[markm]

Seems sensible.

[cem]

As mentioned in email, we might also consider publishing a seed generation to the VDSO page from Fortuna.