Log Message:
Add makeshift NetBSD support using POSIX semaphores.

Not upstreamed because I think NetBSD should be given the chance to
implement some better interprocess synchronization primitives first.
See PR lib/49529.

In particular what's missing is an interprocess sync primitive that can:
  1) be passed safely through MAP_SHARED memory
  2) support atomic unlock-and-block (like pthread_cond_wait)
  3) wake up all waiters at once