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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

arc4random(9): Integrate with RANDOM_FENESTRASX push-reseed

There is no functional change for the existing Fortuna random(4)
implementation, which remains the default in GENERIC.

In the FenestrasX model, when the root CSPRNG is reseeded from pools due to
an (infrequent) timer, child CSPRNGs can cheaply detect this condition and
reseed. To do so, they just need to track an additional 64-bit value in the
associated state, and compare it against the root seed version (generation)
on random reads.

This revision integrates arc4random(9) into that model without substantially
changing the design or implementation of arc4random(9). The motivation is
that arc4random(9) is immediately reseeded when the backing random(4)
implementation has additional entropy. This is arguably most important
during boot, when fenestrasX is reseeding at 1, 3, 9, 27, etc., second
intervals. Today, arc4random(9) has a hardcoded 300 second reseed window.
Without this mechanism, if arc4random(9) gets weak entropy during initial
seed (and arc4random(9) is used early in boot, so this is quite possible),
it may continue to emit poorly seeded output for 5 minutes. The FenestrasX
push-reseed scheme corrects consumers, like arc4random(9), as soon as
possible.

Reviewed by: markm
Approved by: csprng (markm)
Differential Revision: https://reviews.freebsd.org/D22838

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