Define static resend_lost_signals(), restart_all() in pthread_stop_world

Define static resend_lost_signals(), restart_all() in pthread_stop_world
(code refactoring)

Issue #181 (bdwgc).

* pthread_stop_world.c [!GC_OPENBSD_UTHREADS && !NACL && GC_ASSERTIONS]
(suspend_restart_barrier): Check that the count of GC_suspend_ack_sem
is zero at the end of the function.
* pthread_stop_world.c [!GC_OPENBSD_UTHREADS && !NACL]
(resend_lost_signals): New static function (the code is moved from
GC_stop_world).
* pthread_stop_world.c [DEBUG_THREADS] (GC_suspend_all): Move the
assignment of GC_stopping_thread and GC_stopping_pid to GC_stop_world.
* pthread_stop_world.c [!GC_OPENBSD_UTHREADS && !NACL] (GC_stop_world):
Call resend_lost_signals() if GC_retry_signals.
* pthread_stop_world.c [!NACL] (GC_restart_all): New static function
(the code is moved from GC_start_world).
* pthread_stop_world.c [!NACL] (GC_start_world): Declare n_live_threads
local variable; call GC_restart_all.