pgsql: Remove O_FSYNC and associated macros.

Remove O_FSYNC and associated macros.

O_FSYNC was a pre-POSIX way of spelling O_SYNC, supported since commit
9d645fd84c3 for non-conforming operating systems of the time. It's not
needed on any modern system. We can just use standard O_SYNC directly
if it exists (= all targeted systems except Windows), and get rid of our
OPEN_SYNC_FLAG macro.

Similarly for standard O_DSYNC, we can just use that directly if it
exists (= all targeted systems except DragonFlyBSD), and get rid of our
OPEN_DATASYNC_FLAG macro.

We still avoid choosing open_datasync as a default value for
wal_sync_method if O_DSYNC has the same value as O_SYNC (= only
OpenBSD), so there is no change in default behavior.

Discussion: https://postgr.es/m/CA%2BhUKGJE7y92NY7FG2ftUbZUaqohBU65_Ys_7xF5mUHo4wirTQ%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/a1b56090eb54544b8ae01b95b424c254c6b71678

Modified Files
--------------
src/backend/access/transam/xlog.c | 20 ++++++++++----------
src/bin/pg_test_fsync/pg_test_fsync.c | 12 ++++++------
src/include/access/xlogdefs.h | 22 +++-------------------
3 files changed, 19 insertions(+), 35 deletions(-)