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.

src/lib/empthread/lwp.c

@@ -101,13 +101,13 @@ empth_select(int fd, int flags)
 void
 empth_wakeup(empth_t *a)
 {
-    lwpWakeupFd(a);
+    lwpWakeup(a);
 }
 
-void
+int
 empth_sleep(time_t until)
 {
-    lwpSleepUntil(until);
+    return lwpSleepUntil(until);
 }
 
 int