SIGEV_THREAD timers: re-block signals when reusing kernel thread

previously, we left any changes made by the application to the timer
thread's signal mask active when resetting the thread state for reuse.
not only did this violate the intended invariant that timer threads
start with all signals blocked; it also allowed application code to
execute in a thread that, formally, did not exist. and further, if the
internal SIGTIMER signal became unblocked, it could also lead to
missed timer expiration events.

src/time/timer_create.c

@@ -30,6 +30,8 @@ static void cleanup_fromsig(void *p)
 {
 	pthread_t self = __pthread_self();
 	__pthread_tsd_run_dtors();
+	__block_app_sigs(0);
+	__syscall(SYS_rt_sigprocmask, SIG_BLOCK, SIGTIMER_SET, 0, _NSIG/8);
 	self->cancel = 0;
 	self->cancelbuf = 0;
 	self->canceldisable = 0;