semaphores in the runtime

semaphores in the runtime

  Trivial refactorings:

  * Rename STATE_SUSPENDED STATE_STOPPED for elegance. (Spells with the same
    number of letters as STATE_RUNNING, things line up nicer.)

  * Re-express make_fixnum in terms of MAKE_FIXNUM so that we can use the
    latter to define STATE_* names in a manner acceptable to use in
    switch-statements.

  * Move Mach exception handling initialization to darwin_init from
    create_initial_thread so that current_mach_task gets initialized before
    the first thread struct is initialized.

  The Beef:

    Replace condition variables in the runtime with semaphores.

    On most platforms use sem_t, but on Darwin use semaphore_t. Hide the
    difference behind, os_sem_t, os_sem_init, os_sem_destroy, os_sem_post, and
    os_sem_wait.

    POSIX realtime semaphores are supposedly safe to use in signal handlers,
    unlike condition variables -- and experimentally at least Mach semaphores
    on Darwin are a lot less prone to problems.

    (Our pthread mutex usage isn't quite kosher either, but it's the
    pthread_cond_wait and pthread_cond_broadcast pair that seemed to be
    causing most of the trouble.)