MFC r326276:

various: general 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.

No functional change intended.

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Make c99(1) invoke /usr/bin/cc with argv[0] set to "/usr/bin/cc" instead
of just "cc", since there is no reason to cause additional path searches
in this case.

MFC after: 3 days

Add missing static keywords to c99(1)

Sync program's usage() with manpage's SYNOPSIS.

Fix off-by-one error.

Re-add the code which maps POSIX standard library names into the ones
FreeBSD uses; f.e. -lpthread -> -pthread, -lxnet -> nothing.

Add the c99(1) utility, which is nothing more than a synonym for
cc -std=iso9899:1999 -pedantic, and is required by SUSv3.

PR; 36087