Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

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

Set up the /usr/tests hierarchy.

This is a MFC of the following into stable/10:
- r257097 Set up the /usr/tests hierarchy.
- r257098 Add missing WITHOUTTESTS file.
- r257100 Add a tests(7) manual page.
- r257105 Disable WITHTESTS= for now.
- r257848 Fix buildworld when WITHTESTS is enabled.
- r257850 Subsume the functionality of MKATF into MKTESTS.
- r257851 Handle the removal of the test suite when WITHOUTTESTS=yes.
- r257852 Install category Kyuafiles from their category directories.
- r258232 Install BSD.tests.mtree when MKTESTS is yes.

Note that building with WITH_TESTS is still broken at this point (and
hence why WITHOUT_TESTS is the set as the default). Subsequent pullups
will fix the remaining issues.

Set up the /usr/tests hierarchy.

Populate /usr/tests with the only test programs that currently live
in the tree (those in lib/libcrypt/tests/) and add all the build
machinery to accompany this change.

In particular:

- Add a WITHOUT_TESTS variable that users can define to request that
no tests be put in /usr/tests.
- Add a top-level Kyuafile for /usr/tests and a way to create similar
Kyuafiles in top-level subdirectories.
- Add a BSD.tests.dist file to define the directory layout of
/usr/tests.

Submitted by: Julio Merino jmmv google.com
Reviewed by: sjg
MFC after: 2 weeks