src/drivers: do not call pthread_exit() at the end of the drivers; just return NULL.

This is better if we ever want to run the driver loop in a thread that belongs to the application.

src/drivers/cpu/driver_cpu.c

@@ -238,6 +238,5 @@ void *_starpu_cpu_worker(void *arg)
 
 	_STARPU_TRACE_WORKER_DEINIT_END(_STARPU_FUT_CPU_KEY);
 
-	pthread_exit(NULL);
 	return NULL;
 }

src/drivers/cuda/driver_cuda.c

@@ -376,10 +376,7 @@ void *_starpu_cuda_worker(void *arg)
 
 	_STARPU_TRACE_WORKER_DEINIT_END(_STARPU_FUT_CUDA_KEY);
 
-	pthread_exit(NULL);
-
 	return NULL;
-
 }
 
 void starpu_cublas_report_error(const char *func, const char *file, int line, cublasStatus status)

src/drivers/gordon/driver_gordon.c

@@ -488,5 +488,5 @@ void *_starpu_gordon_worker(void *arg)
 	gordon_deinit();
 	_STARPU_DEBUG("gordon was deinited\n");
 
-	pthread_exit((void *)0x42);
+	return NULL;
 }

src/drivers/opencl/driver_opencl.c

@@ -509,8 +509,6 @@ void *_starpu_opencl_worker(void *arg)
 
         _starpu_opencl_deinit_context(devid);
 
-	pthread_exit(NULL);
-
 	return NULL;
 }