Re: Windows now has fdatasync()

Ok, I've pushed the Windows patch.  I'll watch the build farm to see
if I've broken any of the frankentoolchain Windows animals.

Mikael kindly upgraded conchuela, so that leaves just prairiedog
without fdatasync.  I've attached a patch to drop the configure probe
for that once prairiedog's host is reassigned to new duties, if we're
agreed on that.

While in this part of the code I noticed another anachronism that
could be cleaned up: our handling of the old pre-standard BSD O_FSYNC
flag.  Pulling on that I noticed I could remove a bunch of associated
macrology.