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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

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.

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

Include a couple of headers to ensure consistency between the prototype and
the function definition.

Fix the style of the SCM ID's.
I believe have made all of libc .c's as consistent as possible.

Check if string is not NULL, not *string, before setting *string.

Add __FBSDID()s to libutil

o Rename "namespace" argument to "attrnamespace" as namespace is a C++
reserved word.

Submitted by: jkh
Obtained from: TrustedBSD Project

o To support new EA interface with explicit namespaces, introduce two
utility functions which convert between string namespace names and
numeric constants used by the interface. Right now, two namespaces
are supported, EXTATTR_NAMESPACE_SYSTEM ("system") and
EXTATTR_NAMESPACE_USER ("user"). These functions are used by
various userland EA utilities, rather than hard coding the routines
all over the place.

Obtained from: TrustedBSD Project