* exceptions.cc (_cygtls::handle_SIGCONT): Simplify loop waiting for
	sig_handle_tty_stop to wake up.  Make sure to unlock before calling
	yield to avoid starvation of sig_handle_tty_stop.  Add comments.
	* miscfuncs.cc (yield): Explain why yield should never be called under
	_cygtls::lock conditions.  Call SleepEx with 1ms timeout.  Explain why.
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