lib{c,sys}: return wrapped syscall APIs to libc

These provide standard APIs, but are implemented using another system
call (e.g., pipe implemented in terms of pipe2) or are interposed by the
threading library to support cancelation.

After discussion with kib (see D44111), I've concluded that it is
better to keep most public interfaces in libc with as little
as possible in libsys.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D44241

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/

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

closefrom: clamp lowfd to >= 0; close_range's parameters are unsigned.

Pointy hat: kevans
Reported by: CI (lwhsu)

Mark closefrom(2) COMPAT12, reimplement in libc to wrap close_range

Include a temporarily compatibility shim as well for kernels predating
close_range, since closefrom is used in some critical areas.

Reviewed by: markj (previous version), kib
Differential Revision: https://reviews.freebsd.org/D24399