* bsd_helper.h: Throughout, convert "struct thread" to "class thread".

	* bsd_helper.h: Throughout, convert "struct thread" to "class thread".
	* bsd_mutex.cc (_msleep): Fetch signal_arrived handle from thread's
	ipcblk.
	* process.h (class process): Drop _signal_arrived and align methods.
	(process_cache::process): Drop signal_arrived parameter.
	* process.cc (process::process): Ditto.  Drop related code.
	(process::~process): Drop closing signal_arrived handle.
	(process_cache::process): Drop signal_arrived parameter in call to
	process::process.
	(thread::dup_signal_arrived): New method duplicating thread's
	signal_arrived handle.
	(thread::close_signal_arrived): New method closing thread's
	signal_arrived handle.
	* msg.cc (client_request_msg::serve): Drop signal_arrived parameter from
	call to process_cache::process. Use thread constructor to initialize td.
	* sem.cc (client_request_sem::serve): Ditto.
	* shm.cc (client_request_shm::serve): Ditto.