random(4): Fortuna: allow increased concurrency
    Add experimental feature to increase concurrency in Fortuna.  As this
    diverges slightly from canonical Fortuna, and due to the security
    sensitivity of random(4), it is off by default.  To enable it, set the
    tunable kern.random.fortuna.concurrent_read="1".  The rest of this commit
    message describes the behavior when enabled.

    […]

* https://github.com/freebsd/freebsd-src/commit/179f62805cf05c...

    random(4): Flip default Fortuna generator over to Chacha20
    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 (AES-ICM)
    mode of operation by setting the 'kern.random.use_chacha20_cipher' tunable
    to 0 in loader.conf(5).
    
    Approved by: csprng(delphij, markm)
    Differential Revision: https://reviews.freebsd.org/D22878

* https://github.com/freebsd/freebsd-src/commit/68b97d40fbe826...