sys: Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

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