Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\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.

Merge ^/user/ngie/release-pkg-fix-tests to unbreak how test files are installed
after r298107

Summary of changes:

- Replace all instances of FILES/TESTS with ${PACKAGE}FILES. This ensures that
namespacing is kept with FILES appropriately, and that this shouldn't need
to be repeated if the namespace changes -- only the definition of PACKAGE
needs to be changed
- Allow PACKAGE to be overridden by callers instead of forcing it to always be
`tests`. In the event we get to the point where things can be split up
enough in the base system, it would make more sense to group the tests
with the blocks they're a part of, e.g. byacc with byacc-tests, etc
- Remove PACKAGE definitions where possible, i.e. where FILES wasn't used
previously.
- Remove unnecessary TESTSPACKAGE definitions; this has been elided into
bsd.tests.mk
- Remove unnecessary BINDIRs used previously with ${PACKAGE}FILES;
${PACKAGE}FILESDIR is now automatically defined in bsd.test.mk.
- Fix installation of files under data/ subdirectories in lib/libc/tests/hash
and lib/libc/tests/net/getaddrinfo
- Remove unnecessary .include <bsd.own.mk>s (some opportunistic cleanup)

Document the proposed changes in share/examples/tests/tests/... via examples
so it's clear that ${PACKAGES}FILES is the suggested way forward in terms of
replacing FILES. share/mk/bsd.README didn't seem like the appropriate method
of communicating that info.

MFC after: never probably
X-MFC with: r298107
PR: 209114
Relnotes: yes
Tested with: buildworld, installworld, checkworld; buildworld, packageworld
Sponsored by: EMC / Isilon Storage Division

Remove unnecessary svn:executable property from source file.

The atf cp_test.sh sample file should have never been marked executable in
the first place because this file needs to be "built" first before being
usable.

Add some sample test programs.

This change adds some sample test cases to share/examples/tests/
demonstrating the basic usage of the atf and plain interfaces.

These test programs are fully-functional and are installed as part
of the test suite, which guarantees that the sample code remains
correct. However, they currently mostly serve as a placeholder for
additional examples and may be incomplete (depending on how you
look at them). I will see what else can be useful while working on
documentation.

As a bonus, the addition of these tests exercise the *.test.mk files,
one of which (plain.test.mk) was not yet in use, and also demonstrates
that it's possible to mix different kinds of test programs into the
same test suite.

Approved by: rpaulo (mentor)