sys: Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

Fix a typo.

Reviewed by: kib

sys/x86: further adoption of SPDX licensing ID tags.

Mainly focus on files that use BSD 2-Clause license, however the tool I
was using misidentified 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.

Remove lint support from system headers and MD x86 headers.

Reviewed by: dim, jhb
Discussed with: imp
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D13156

POSIX states that #include <signal.h> shall make both mcontext_t and
ucontext_t available. Our code even has XXX comment about this.

Add a bit of compliance by moving struct __ucontext definition into
sys/_ucontext.h and including it into signal.h and sys/ucontext.h.

Several machine/ucontext.h headers were changed to use namespace-safe
types (like uint64_t->__uint64_t) to not depend on sys/types.h.
struct __stack_t from sys/signal.h is made always visible in private
namespace to satisfy sys/_ucontext.h requirements.

Apparently mips _types.h pollutes global namespace with f_register_t
type definition. This commit does not try to fix the issue.

PR: 207079
Reported and tested by: Ting-Wei Lan <lantw44@gmail.com>
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks

Convert machine/elf.h, machine/frame.h, machine/sigframe.h,
machine/signal.h and machine/ucontext.h into common x86 includes,
copying from amd64 and merging with i386.

Kernel-only compat definitions are kept in the i386/include/sigframe.h
and i386/include/signal.h, to reduce amd64 kernel namespace pollution.
The amd64 compat uses its own definitions so far.

The _MACHINE_ELF_WANT_32BIT definition is to allow the
sys/boot/userboot/userboot/elf32_freebsd.c to use i386 ELF definitions
on the amd64 compile host. The same hack could be usefully abused by
other code too.