Follow-up to #42422:
- Rename process_image() to context_process_image(), since it now
operates on a Context object.
- Use IN_SET() in image_type_can_sysupdate() instead of a switch.
- Name the return parameters of context_list_components() ret_xyz, per
our coding style.
- Drop a redundant "else" after a return in vl_method_check_new().