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/

lib/libc/string/ffs*.c: work around gcc warning

Gcc warns of infinite recursion if we use __builtin_ffs*() to
implement ffs*(). This is because gcc uses ffs() to implement
these on some platforms. Sidestep the warning by using
__builtin_ctz*() for these.

Sponsored by: FreeBSD Foundation
Reported by: jlduran@gmail.com, jhb
Fixes: ee8b0c43 (D40730)
Reviewed by: jhb, mhorne
Approved by: jhb
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D40966

lib/libc/string: replace ffs/fls implementations with clang builtins

Most architectures we support (except for riscv64) have instructions
to compute these functions very quickly. Replace old code with the
ffs and clz builtin functions, allowing clang to generate good code
for all architectures.

As a consequence, toss out arm and i386 ffs() implementations.

Sponsored by: FreeBSD Foundation
Approved by: mhorne
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D40730

General further adoption of SPDX licensing ID tags.

Mainly focus on files that use BSD 3-Clause license.

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.

Special thanks to Wind River for providing access to "The Duke of
Highlander" tool: an older (2014) run over FreeBSD tree was useful as a
starting point.

Renumber clauses to reduce diffs to other versions

NetBSD, OpenBSD, and Android's Bionic number the clauses 1 through 3,
so follow suit to make comparison easier.

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 the ffsll and flsll functions. These are ffs and fls operating
on long long arguments.

Reviewed by: bde (previous version, that included asm implementation
for all ffs and fls functions on i386 and amd64)
MFC after: 2 weeks