Refactor: Core, give the IDWALK_… enum an explicit name

Give the `IDWALK_…` enum an explicit name: `LibraryForeachIDFlag`. This way
the flags are type-safe, and it's known where values come from. This is
much preferred (at least by me) to just having `int flags`.

Uses of `0` have been replaced with `IDWALK_NOP` as that has the same value
and is of the right type.

One invalid use of `IDWALK_CB_NOP` was detected by this change, and is
replaced by `IDWALK_NOP`.

This change might be incomplete; I gave the enum a name, and then fixed
the compiler errors.

No functional changes.

Pull Request: https://projects.blender.org/blender/blender/pulls/131865

source/blender/python/intern/bpy_rna_id_collection.cc

@@ -247,7 +247,7 @@ static PyObject *bpy_user_map(PyObject * /*self*/, PyObject *args, PyObject *kwd
 
       data_cb.id_curr = id;
       BKE_library_foreach_ID_link(
-          nullptr, id, foreach_libblock_id_user_map_callback, &data_cb, IDWALK_CB_NOP);
+          nullptr, id, foreach_libblock_id_user_map_callback, &data_cb, IDWALK_NOP);
 
       if (data_cb.py_id_curr) {
         Py_DECREF(data_cb.py_id_curr);