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