Clear pending signals targeting exiting thread

Clear pending signals targeting exiting thread

	* cygtls.cc (_cygtls::remove): Call remove_pending_sigs.
	* cygtls.h (_cygtls::remove_pending_sigs): Declare.
	* sigproc.cc (pending_signals::clear): Define new method taking a
	_cygtls pointer argument.  Drop pending signals for that thread.
	(_cygtls::remove_pending_sigs): Call pending_signals::clear for this
	thread.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>