We define in detail the X/Open System Interfaces i.e. _XOPEN_UNIX
and all of the X/Open System Interfaces (XSI) Options Groups.
The XSI options groups include encryption, realtime, advanced
realtime, realtime threads, advanced realtime threads, tracing,
streams, and legacy interfaces.
Signed-off-by: Carlos O'Donell <carlos@redhat.com> Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>