libc: Rewrite quick_exit() and at_quick_exit() using C11 atomics.

Compiler memory barriers do not prevent the CPU from executing the code
out of order. Switch to C11 atomics. This also lets us get

libc: Rewrite quick_exit() and at_quick_exit() using C11 atomics.

Compiler memory barriers do not prevent the CPU from executing the code
out of order. Switch to C11 atomics. This also lets us get rid of the
mutex; instead, loop until the compare_exchange succeeds.

While here, change the return value of at_quick_exit() on failure to
the more traditional -1, matching atexit().

Sponsored by: Klara, Inc.
Reviewed by: Olivier Certner, kevans, kib
Differential Revision: https://reviews.freebsd.org/D41936

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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

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libc: further adoption of SPDX licensing ID tags.

Mainly focus on files that use BSD 2-Clause license, however the tool I
was using mis-identified many licenses so this was mostly a manual - error
p

libc: further adoption of SPDX licensing ID tags.

Mainly focus on files that use BSD 2-Clause license, however the tool I
was using mis-identified many licenses so this was mostly a manual - error
prone - task.

The Software Package Data Exchange (SPDX) group provides a specification
to make it easier for automated tools to detect and summarize well known
opensource licenses. We are gradually adopting the specification, noting
that the tags are considered only advisory and do not, in any way,
superceed or replace the license texts.

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Merge ^/head r323559 through r325504.

MFhead@r325422

C++17 requires quick_exit(3) to be async-signal safe.

Make it safe, and update man page with the useful information.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week

IFC @230489 (pending review).

IFC to head to catch up the bhyve branch

Approved by: grehan@

Small style(9) improvements.

Approved by: dim (mentor)

style(9) cleanups.

Approved by: brooks (mentor)

Implement quick_exit() / at_quick_exit() from C++11 / C1x. Also add a
__noreturn macro and modify the other exiting functions to use it.

The __noreturn macro, unlike __dead2, must be used BEFORE th

Implement quick_exit() / at_quick_exit() from C++11 / C1x. Also add a
__noreturn macro and modify the other exiting functions to use it.

The __noreturn macro, unlike __dead2, must be used BEFORE the function.
This is in line with the C and C++ specifications that place _Noreturn (c1x)
and [[noreturn]] (C++11) in front of the functions. As with __dead2, this
macro falls back to using the GCC attribute.

Unfortunately, clang currently sets the same value for the C version macro
in C99 and C1x modes, so these functions are hidden by default. At some
point before 10.0, I need to go through the headers and clean up the C1x /
C++11 visibility.

Reviewed by: brooks (mentor)

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