Fix a bug where a thread that hit the race where the sleep timeout fires

while the thread does not hold the thread lock would stop blocking for
subsequent interruptible sleeps and would always immediately fail the
sleep with EWOULDBLOCK instead (even sleeps that didn't have a timeout).

Some background:
- KSE has a facility for allowing one thread to interrupt another thread.
  During this process, the target thread aborts any interruptible sleeps
  much as if the target thread had a pending signal.  Once the target
  thread acknowledges the interrupt, normal sleep handling resumes.  KSE
  manages this via the TDF_INTERRUPTED flag.  Specifically, it sets the
  flag when it sends an interrupt to another thread and clears it when
  the interrupt is acknowledged.  (Note that this is purely a software
  interrupt sort of thing and has no relation to hardware interrupts
  or kernel interrupt threads.)
- The old code for handling the sleep timeout race handled the race
  by setting the TDF_INTERRUPT flag and faking a KSE-style thread
  interrupt to the thread in the process of going to sleep.  It probably
  should have just checked the TDF_TIMEOUT flag in sleepq_catch_signals()
  instead.
- The bug was that the sleepq code would set TDF_INTERRUPT but it was
  never cleared.  The sleepq code couldn't safely clear it in case there
  actually was a real KSE thread interrupt pending for the target thread
  (in fact, the sleepq timeout actually stomped on said pending interrupt).
  Thus, any future interruptible sleeps (*sleep(.. PCATCH ..) or
  cv_*wait_sig()) would see the TDF_INTERRUPT flag set and immediately
  fail with EWOULDBLOCK.  The flag could be cleared if the thread belonged
  to a KSE process and another thread posted an interrupt to the original
  thread.  However, in the more common case of a non-KSE process, the
  thread would pretty much stop sleeping.
- Fix the bug by just setting TDF_TIMEOUT in the sleepq timeout code and
  not messing with TDF_INTERRUPT and td_intrval.  With yesterday's fix to
  fix sleepq_switch() to check TDF_TIMEOUT, this is now sufficient.

MFC after:	3 days

sys/kern/subr_sleepqueue.c

@@ -817,8 +817,7 @@ sleepq_timeout(void *arg)
 	 * sleepq_catch_signals().
 	 */
 	if (TD_ON_SLEEPQ(td)) {
-		td->td_flags |= TDF_TIMEOUT | TDF_INTERRUPT;
-		td->td_intrval = EWOULDBLOCK;
+		td->td_flags |= TDF_TIMEOUT;
 		thread_unlock(td);
 		return;
 	}