Remove $FreeBSD$: one-line .c comment pattern

Remove /^/[*/]\s*\$FreeBSD\$.*\n/

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.

Add __BIT and __BITS macros from NetBSD to help support new testcases

MFC after: 1 week

When compiling on macOS or Linux, __dead can be defined already.
Conditionally define __dead.

The NetBSD libc tests use several definitions/macros that aren't available in
FreeBSD

Add the missing compat definitions/macros to lib/libnetbsd so the testcases
can be compiled with libnetbsd without having to invent ad hoc #define's, or
having to convert things over to FreeBSD idioms

Reviewed by: brooks
Phabric: D993
Sponsored by: EMC / Isilon Storage Division

Add libnetbsd, a thin compatibility layer intended to allow a limited
set of NetBSD software to compile as part of the FreeBSD build with
little or no modifiction. It is built as a static library and not
installed for general use. Likewise, its header files are not
installed.

Sponsored by: DARPA, AFRL