Cleanup and do minor refactoring before converting testcases to ATF

Cleanup and do minor refactoring before converting testcases to ATF

- Convert errx(-1, ..) to errx(1, ..)
- Move the aio(4) checks to a single function (aio_available); use modfind(2)
  instead of depending on SIGSYS (doesn't work when aio(4) support is missing,
  not documented in the aio syscall manpages).
- Use aio_available liberally in the testcase functions
- Use mkstemp(3) + unlink(2) instead of mktemp(3)
- Fix some -Wunused warnings
- Bump WARNS to 6

MFC after: 1 week
Submitted by: mjohnston [*]
Sponsored by: EMC / Isilon Storage Division