reject invalid attribute settings

note that this is a pedantic conformance issue and waste of code. it
only affects broken code or code that is probing for conformance.

src/thread/pthread_attr_setscope.c

@@ -2,5 +2,6 @@
 
 int pthread_attr_setscope(pthread_attr_t *a, int scope)
 {
+	if (scope > 1U) return EINVAL;
 	return 0;
 }