Add a safepoint-based mechanism to avoid SIGALRM for the TIMER facility

Add a safepoint-based mechanism to avoid SIGALRM for the TIMER facility

  - Retrofits the signal-free timer thread for Windows (thanks to
    Anton Kovalenko) to POSIXy platforms.

  - Provide os_* functions in the C runtime which simulate the win32
    API for waitable timers.

Currently supported on Linux (timerfd), FreeBSD (kqueue), and SunOS
(completion ports).  A tentative (untested) implementation is
provided for Darwin's kqueue.