Coverity CID 1616018
O_SYNC and O_DSYNC have the same value which led Coverity to note that
checking for both values in if's or switches leads to dead code. The
solution is to add a cpp check that they are equal and enough commentary
so if they ever are not the same, there is a hint as to why the cpp
check got tripped.
Closes#5100.
f29986391d