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.

Modified Files:
--------------
pgsql/doc/src/sgml:
runtime.sgml (r1.310 -> r1.311)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/runtime.sgml.diff?r1=1.310&r2=1.311)
pgsql/src/backend/access/transam:
xlog.c (r1.181 -> r1.182)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c.diff?r1=1.181&r2=1.182)
pgsql/src/backend/utils/misc:
postgresql.conf.sample (r1.137 -> r1.138)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/postgresql.conf.sample.diff?r1=1.137&r2=1.138)
pgsql/src/include/port:
win32.h (r1.43 -> r1.44)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/port/win32.h.diff?r1=1.43&r2=1.44)
pgsql/src/port:
open.c (r1.8 -> r1.9)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/port/open.c.diff?r1=1.8&r2=1.9)