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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

sysctl: use names instead of magic numbers.

Replace magic numbers with symbols for internal sysctl operations.
Convert in-kernel and libc consumers.

Submitted by: Pawel Biernacki
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D21693

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.

A new jail(8) with a configuration file, to replace the work currently done
by /etc/rc.d/jail.

Create the altix project branch. The altix project will add support
for the SGI Altix 350 to FreeBSD/ia64. The hardware used for porting
is a two-module system, consisting of a base compute module and a
CPU expansion module. SGI's NUMAFlex architecture can be an excellent
platform to test CPU affinity and NUMA-aware features in FreeBSD.

Add proper const keywords to sysctl(3) parameters.

The `name' and `newp' arguments can be marked const, because the buffers
they refer to are never changed. While there, perform some other
cleanups:

- Remove K&R from sysctl.c.
- Implement sysctlbyname() using sysctlnametomib() to prevent
duplication of an undocumented kernel interface.
- Fix some whitespace nits.

It seems the prototypes are now in sync with NetBSD as well.

- Import the HEAD csup code which is the basis for the cvsmode work.

#include <string.h> for strlen()'s prototype.

Fix SCM ID's.

Suppress compiler warning by adding a type cast.

Add the function sysctlnametomib to libc. Details on the semantics
and use of this function have been added to the sysctl.3 manual page.