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

MFC various fixes for the ATF tests.

- r260505 Allow tests to provide a Kyuafile when they relied on auto-generation.
- r260525 Respect the original layout of the atf-{c,c++} tests.
- r260526 Fix path to the process_helpers for the libatf-c++ tests.
- r260576 Generate and install pkg-config files for atf.
- r260577 Add atf pkg-config files from the vendor branch.
- r260584 Prevent misc_helpers from running as a test.

Generate and install pkg-config files for atf.

These files are required to get packages in ports to build against atf and
also to get a couple of currently-failing tests to pass.

I'm following the approach already used by the libusb pkg-config files
installed by the system regarding the location and the install rules.

MFC after: 5 days