Re: Direct I/O

Thanks both for looking, and thanks for the explanation Ilmari.
Pushed with your improvements.  The 4 CI systems run the tests
(Windows and Mac by special always-expected-to-work case, Linux and
FreeBSD by successful pre-flight perl test of O_DIRECT), and I also
tested three unusual systems, two that skip for different reasons and
one that will henceforth run this test on the build farm so I wanted
to confirm locally first:

Linux/tmpfs: 1..0 # SKIP pre-flight test if we can open a file with
O_DIRECT failed: Invalid argument
OpenBSD:     t/004_io_direct.pl .............. skipped: no O_DIRECT
illumos:     t/004_io_direct.pl .............. ok

(Format different because those last two are autoconf, no meson on my
collection of Vagrant images yet...)