>Ideally, if a program is multithreaded (using our pthreads, once we
>have it) applications and other libraries will not be allowed to use
>these timers/signals.

If that's the goal, does it help to put them in the
reserved-to-implementation namespace: _SIGTHREAD, _SIGTHREADV,
_ITIMER_THREADV, _ITIMER_VIRTUAL?

Then, if anyone beside netbsd library implementors uses them,
its even more clearly their own fault :)