O_RSYNC and O_DSYNC

[Joel Sherrill]

Hi

Looking through old RTEMS PRs, someone reported these
are missing in newlib. If no target has needed them
should we add them for completion?

If so, Linux has them wrapped like this:

#if __USE_POSIX1999309 || defined __USE_UNIX98
   #define O_DSYNC XXX
   #if defined(__O_RSYNC)
     #define O_RSYNC __ORSYNC
#else
     #define O_RSYNC XXX
#endif
#endif

How/if should we guard the definitions of O_RSYNC/O_DSYNC?
Open Group just cites them as C Library extensions. Linux is
marking is as 1999. Does that correspond to newlib's
__XSI_VISIBLE >= 600?

Should I just pick the next logical values in sys/_default_fcntl.h
around line 25 and add these around 45?

Thanks.

--joel