Add sysexits.h from FreeBSD r259958

I'd need this for getconf.
Add getconf(1) from FreeBSD r259958

This one lacks -a but is simpler to port than the (e)glibc one,
and should be way enough for usage patterns like counting CPUs,
cf. https://gist.github.com/jj1bdx/5746298

TODO: Handle message files?
TODO: Install manpage?
Mention getconf in FreeBSD sections in the credits.
Add getconf to the image
Provide cmd:getconf in haiku package