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>

winsup/cygwin/cygtls.h

@@ -267,6 +267,7 @@ public:
   void handle_SIGCONT ();
 private:
   void __reg3 call2 (DWORD (*) (void *, void *), void *, void *);
+  void remove_pending_sigs ();
   /*gentls_offsets*/
 };
 #pragma pack(pop)