MFC r292822:

Remove retval to fix a -Wunused-but-set-variable warning from gcc 4.9

MFC r290914:

Integrate tools/regression/pipe in to the FreeBSD test suite as
tests/sys/kern/pipe

- Fix style(9) bugs
- Fix compiler warnings
- Use `nitems(x)` instead of `sizeof(x) / sizeof(*x)` pattern

The testcases will be converted over to ATF eventually, but for now will be
integrated in as plain C tests

Sponsored by: EMC / Isilon Storage Division

Integrate tools/regression/pipe in to the FreeBSD test suite as
tests/sys/kern/pipe

- Fix style(9) bugs
- Fix compiler warnings
- Use `nitems(x)` instead of `sizeof(x) / sizeof(*x)` pattern

The testcases will be converted over to ATF eventually, but for now will be
integrated in as plain C tests

MFC after: 1 week
Sponsored by: EMC / Isilon Storage Division

Add two more pipe regression tests. Not yet hooked into the test framework.