This macro makes uses the __sys_<foo>_t typedefs from libsys.h to
greatly simplify calling functions in the interposing table.

Reviewed by:	kib
Differential Revision:	https://reviews.freebsd.org/D44389

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

Remove core system call implementations and documentation to lib/libsys
and lib/libsys/<arch> from lib/libc/sys and lib/libc/<arch>/<sys>.
Update paths to allow libc to find them in their new home.

Reviewed by:	kib, emaste, imp
Pull Request:	https://github.com/freebsd/freebsd-src/pull/908

System calls or their wrappers are now interposed by
__libsys_interposing with purely libc entries remaining in
__libc_interposing.

Use __libsys_interposing_slot in libthr to update __libsys_interposing,
but also make __libc_interposing_slot fall back to
__libsys_interposing_slot so an out of date libc has a chance of working
during updates.

Reviewed by:	kib, emaste, imp
Pull Request:	https://github.com/freebsd/freebsd-src/pull/908

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

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

This has already been done for most files that have the Foundation as
the only listed copyright holder.  Do it now for files that list
multiple copyright holders, but have the Foundation copyright in its own
section.

Sponsored by:	The FreeBSD Foundation

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.

pwrite(2) syscalls are wrapped to provide compatibility with pre-7.x
kernels which required padding before the off_t parameter.  The
fcntl(2) contains compatibility code to handle kernels before the
struct flock was changed during the 8.x CURRENT development.  The
shims were reasonable to allow easier revert to the older kernel at
that time.

Now, two or three major releases later, shims do not serve any
purpose.  Such old kernels cannot handle current libc, so revert the
compatibility code.

Make padded syscalls support conditional under the COMPAT6 config
option.  For COMPAT32, the syscalls were under COMPAT6 already.

Remove WITHOUT_SYSCALL_COMPAT build option, which only purpose was to
(partially) disable the removed shims.

Reviewed by:	jhb, imp (previous versions)
Discussed with:	peter
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week