qpkg: give ID2S macro a more "offical" status

- prereq.c (ID2S), id.h (ID2PF): renamed ID2S to ID2PF and moved it to id.h
- prereq.c (push, satisfies, resolve): change ID2S to ID2PF

qpkg/id.h

@@ -59,6 +59,14 @@ struct id {
 };
 
 
+/*
+ * Helper macro for printing identifiers.
+ * Use with "... %.*s ..."
+ */
+
+#define	ID2PF(id)	(int) (id)->len, (id)->s
+
+
 int comp_id(const void *a, const void *b);
 
 struct tree *make_tree(int (*comp)(const void *a, const void *b));