Make empth_wakeup() and empth_terminate() wake up empth_sleep(), and

Make empth_wakeup() and empth_terminate() wake up empth_sleep(), and
empth_sleep() return whether that happened:
[EMPTH_LWP] (lwpWakeupSleep): New, factored out of lwpSelect().
[EMPTH_LWP] (lwpSelect): Use it.
[EMPTH_LWP] (lwpWakeup): New.  Call lwpWakeupFd() if sleeping in
lwpSleepFd(), lwpWakeupSleep() if sleeping in lwpSleepUntil().
[EMPTH_LWP] (lwpTerminate, empth_wakeup): Use it rather than
lwpWakeupFd().
[EMPTH_LWP] (lwpWakeupFd): Internal linkage.
[EMPTH_LWP] (lwpSleepUntil): Reset member runtime, so that lwpWakeup()
can test it reliably.  Return how sleep woke up.
[EMPTH_LWP] (empth_sleep): Return value of lwpSleepUntil().
[EMPTH_POSIX] (EMPTH_INTR): New.
[EMPTH_POSIX] (empth_wakeup): Set state to it.
[EMPTH_POSIX] (empth_restorectx): Clear state.
[EMPTH_POSIX] (empth_sleep): Don't re-seleep when state is not clear,
i.e. thread was woken up prematurely.  Return how sleep woke up.
[EMPTH_W32] (empth_sleep): Implement by waiting on hThreadEvent with a
timeout rather than a straight Sleep().  Return how sleep woke up.