primitive-exit doesn't _exit

[Andrew Pimlott]

I thought that primitive-exit would call _exit(2) instead of
exit(3).  The reason is basically the same as for not calling scheme
unwind handlers: some library may have registered an atexit hook,
and you don't want this called in the forked process.

I think there needs to be some way to _exit.  primitive-exit seems
like the natural way to me, but if not there, perhaps in a new
function.

Andrew