Property definitions

pthreads $ NativePthreadAttr :: new
	new `{
		pthread_attr_t attr;
		int r = pthread_attr_init(&attr);
		if (r == 0) {
			pthread_attr_t *pattr = malloc(sizeof(pthread_attr_t));
			memmove(pattr, &attr, sizeof(pthread_attr_t));
			return pattr;
		}
		return NULL;
	`}
lib/pthreads/pthreads.nit:181,2--190,3