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I'm convinced. And i've rewritten the patch.
Now fsync(2) is always called first *regardless* of the new
optional argument. The documentation is (a little bit) better
now. And i've added support for NetBSD, AIX and Linux; though:
- for AIX i'm testing (O_SYNC && O_DSYNC) which is almost
 definitely the wrong test for this, but (a) i don't know when
 fsync_range() has been introduced (seems to be many years) and
 (b) i've never really worked on AIX.
 (I only have the documentation:
 http://www.filibeto.org/unix/aix/lib/rel/5.3/basetrf1.pdf.)
- i've added sync_file_range() on Linux because of
 SYNC_FILE_RANGE_WAIT_AFTER in the hope that it means something
 even though the manual page says something else - but Linux and
 documentation is something by itself.
 http://lwn.net/Articles/178199/ states
 Providing all three flags guarantees that those pages are
 actually on disk when the call returns.
- it seems OpenBSD, FreeBSD, Solaris 11 and HP/UX provide data
 reliability through fsync(2) alone, see e.g. the notes near EOF
 of the following Solaris 11 file:
 http://www.filibeto.org/sun/lib/solaris11-express-docs/2010.11/E19963_01/html/821-1464/fcntl.h-3head.html#fcntl.h-3head