random(4): Flip default Fortuna generator over to Chacha20

[cem]

Summary

The implementation was landed in r344913 and has had some bake time (at
least on my personal systems). There is some discussion of the motivation
for defaulting to this cipher as a PRF in the commit log for r344913.

As documented in that commit, administrators can retain the prior mode of
operation by setting the 'kern.random.use_chacha20_cipher' tunable to 0 in
loader.conf(5).

[cem]

Update the surrounding comments to match the modified default, as well.

[markm]

No-brainer.

[delphij]

sys/dev/random/hash.c

82 ↗

(On Diff #65818)

[OPTIONAL] I think eventually AES-ICM would be gone, it's probably better described as "old default", but I'm fine with the current change as-is too.

[cem]

sys/dev/random/hash.c

82 ↗

(On Diff #65818)

I prefer to leave the documented options explicit for now; when and if we intend to remove AES-ICM, we will just remove the knob entirely, along with its documentation.