libc: Purge unneeded cdefs.h

These sys/cdefs.h are not needed. Purge them. They are mostly left-over
from the $FreeBSD$ removal. A few in libc are still required for macros
that cdefs.h defines. Keep those.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D42385

Remove $FreeBSD$: one-line .c pattern

Remove /^[\s*]*__FBSDID\("\$FreeBSD\$"\);?\s*\n/

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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

Update userland arc4random() with OpenBSD's Chacha20 based arc4random().

ObsoleteFiles.inc:

Remove manual pages for arc4random_addrandom(3) and
arc4random_stir(3).

contrib/ntp/lib/isc/random.c:
contrib/ntp/sntp/libevent/evutil_rand.c:

Eliminate in-tree usage of arc4random_addrandom().

crypto/heimdal/lib/roken/rand.c:
crypto/openssh/config.h:

Eliminate in-tree usage of arc4random_stir().

include/stdlib.h:

Remove arc4random_stir() and arc4random_addrandom() prototypes,
provide temporary shims for transistion period.

lib/libc/gen/Makefile.inc:

Hook arc4random-compat.c to build, add hint for Chacha20 source for
kernel, and remove arc4random_addrandom(3) and arc4random_stir(3)
links.

lib/libc/gen/arc4random.c:

Adopt OpenBSD arc4random.c,v 1.54 with bare minimum changes, use the
sys/crypto/chacha20 implementation of keystream.

lib/libc/gen/Symbol.map:

Remove arc4random_stir and arc4random_addrandom interfaces.

lib/libc/gen/arc4random.h:

Adopt OpenBSD arc4random.h,v 1.4 but provide _ARC4_LOCK of our own.

lib/libc/gen/arc4random.3:

Adopt OpenBSD arc4random.3,v 1.35 but keep FreeBSD r114444 and
r118247.

lib/libc/gen/arc4random-compat.c:

Compatibility shims for arc4random_stir and arc4random_addrandom
functions to preserve ABI. Log once when called but do nothing
otherwise.

lib/libc/gen/getentropy.c:
lib/libc/include/libc_private.h:

Fold __arc4_sysctl into getentropy.c (renamed to arnd_sysctl).
Remove from libc_private.h as a result.

sys/crypto/chacha20/chacha.c:
sys/crypto/chacha20/chacha.h:

Make it possible to use the kernel implementation in libc.

PR: 182610
Reviewed by: cem, markm
Obtained from: OpenBSD
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D16760