MFC r280893,r280894,r280895:

r280893:

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

Submitted by: mjohnston [*]
Sponsored by: EMC / Isilon Storage Division

r280894:

Minor cleanup before converting to ATF testcases

• Remove blank (tab-only) lines.
• Fix -Wunused warnings.
• Bump up to WARNS= 6

r280895:

• Fix -Wsign issue
• Bump up to WARNS=6