reject invalid arguments to pthread_barrierattr_setpshared

this is a POSIX requirement.

src/thread/pthread_barrierattr_setpshared.c

@@ -2,6 +2,7 @@
 
 int pthread_barrierattr_setpshared(pthread_barrierattr_t *a, int pshared)
 {
+	if (pshared > 1U) return EINVAL;
 	a->__attr = pshared ? INT_MIN : 0;
 	return 0;
 }