wal_sync_method=fsync_writethrough

Hi,

We allow $SUBJECT on Windows. I'm not sure exactly how we finished up
with that, maybe a historical mistake, but I find it misleading today.
Modern Windows flushes drive write caches for fsync (= _commit()) and
fdatasync (= FLUSH_FLAGS_FILE_DATA_SYNC_ONLY). In fact it is possible
to tell Windows to write out file data without flushing the drive
cache (= FLUSH_FLAGS_NO_SYNC), but I don't believe anyone is
interested in new weaker levels. Any reason not to just get rid of
it?

On macOS, our fsync and fdatasync levels *don't* flush drive caches,
because those system calls don't on that OS, and they offer a weird
special fcntl, so there we offer $SUBJECT for a good reason. Now that
macOS 10.2 systems are thoroughly extinct, I think we might as well
drop the configure probe, though, while we're doing a lot of that sort
of thing.

The documentation also says a couple of things that aren't quite
correct about wal_sync_level. (I would also like to revise other
nearby outdated paragraphs about volatile write caches, sector sizes
etc, but that'll take some more research.)