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

Log Message:
-----------
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.

Tags:
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REL8_0_STABLE

Modified Files:
--------------
pgsql/doc/src/sgml:
runtime.sgml (r1.301.4.3 -> r1.301.4.4)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/runtime.sgml.diff?r1=1.301.4.3&r2=1.301.4.4)
pgsql/src/backend/access/transam:
xlog.c (r1.180 -> r1.180.4.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c.diff?r1=1.180&r2=1.180.4.1)
pgsql/src/backend/utils/misc:
postgresql.conf.sample (r1.134 -> r1.134.4.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/postgresql.conf.sample.diff?r1=1.134&r2=1.134.4.1)
pgsql/src/include/port:
win32.h (r1.42.4.1 -> r1.42.4.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/port/win32.h.diff?r1=1.42.4.1&r2=1.42.4.2)
pgsql/src/port:
open.c (r1.7.4.1 -> r1.7.4.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/port/open.c.diff?r1=1.7.4.1&r2=1.7.4.2)