Add "Fenestras X" alternative /dev/random implementation

Fortuna remains the default; no functional change to GENERIC.

Big picture:

• Scalable entropy generation with per-CPU, buffered local generators.
• "Push" system for reseeding child generators when root PRNG is reseeded. (Design can be extended to arc4random(9) and userspace generators.)
• Similar entropy pooling system to Fortuna, but starts with a single pool to quickly bootstrap as much entropy as possible early on.
• Reseeding from pooled entropy based on time schedule. The time interval starts small and grows exponentially until reaching a cap. Again, the goal is to have the RNG state depend on as much entropy as possible quickly, but still periodically incorporate new entropy for the same reasons as Fortuna.

Notable design choices in this implementation that differ from those
specified in the whitepaper:

• Blake2B instead of SHA-2 512 for entropy pooling
• Chacha20 instead of AES-CTR DRBG
• Initial seeding. We support more platforms and not all of them use loader(8). So we have to grab the initial entropy sources in kernel mode instead, as much as possible. Fortuna didn't have any mechanism for this aside from the special case of loader-provided previous-boot entropy, so most of these sources remain TODO after this commit.

Reviewed by: markm
Approved by: csprng (markm)
Differential Revision: https://reviews.freebsd.org/D22837