Change Win32 O_SYNC method to O_DSYNC because that is what the method

Bruce Momjian <bruce@momjian.us>

Commit: 291cd954fbfa48d8db79f22ffa8d596c837f7ee3
Author: Bruce Momjian <bruce@momjian.us>
Date: 2005-03-24T04:37:07Z
Releases: 8.0.2
Change Win32 O_SYNC method to O_DSYNC because that is what the method
currently does.  This is now the default Win32 wal sync method because
we perfer o_datasync to fsync.

Also, change Win32 fsync to a new wal sync method called
fsync_writethrough because that is the behavior of _commit, which is
what is used for fsync on Win32.

Backpatch to 8.0.X.

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