normalize/unify system objects and how they relate to regular objects

[m-mcgowan]

Currently there's an extra set of commands for handling system objects vs handling regular objects in a profile.

• read system object
• write system object
• write masked system object
• (list system objects) - this doesn't exist but should.

This adds duplication at all levels, which seems unnecessary. We can unify this to make system objects seem less ad-hoc.

The key distinction between system objects and regular objects is that system objects are pre-created by the system and always exist. There is no create/delete support. Currently there's no enumeration support either (to retrieve the id_chain and type) but that will be added - see #4.)