py: Implement keyword-only args.

Implements 'def f(*, a)' and 'def f(*a, b)', but not default
keyword-only args, eg 'def f(*, a=1)'.

Partially addresses issue #524.
diff --git a/py/scope.c b/py/scope.c
index 391ad5d..1f92729 100644
--- a/py/scope.c
+++ b/py/scope.c
@@ -236,7 +236,7 @@
     printf("\n");
     */
     printf("     flags %04x\n", s->scope_flags);
-    printf("     argcount %d\n", s->num_params);
+    printf("     argcount %d\n", s->num_pos_args);
     printf("     nlocals %d\n", s->num_locals);
     printf("     stacksize %d\n", s->stack_size);
 }