Merge r357222 from the clang1000-import branch:

Fix the following -Werror warning from clang 10.0.0 in rbootd:

libexec/rbootd/rmpproto.c:335:49: error: multiple unsequenced modifications to 'filename' [-Werror,-Wunsequenced]
        filename = (filename = strrchr(filepath,'/'))? ++filename: filepath;
                 ~                                     ^

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.

Sponsored by:	Dell EMC Isilon

Off by default, build behaves normally.
WITH_META_MODE we get auto objdir creation, the ability to
start build from anywhere in the tree.

Still need to add real targets under targets/ to build packages.

Differential Revision:       D2796
Reviewed by: brooks imp

permitted by the University of Berkeley on July 22, 1999.

Reviewed by:	imp
MFC after:	1 week