cache-defaults
--------------

Cache variables now have :prop_cache:`DEFAULT` and :prop_cache:`DEFAULTED`
properties. These properties may be used to offer cache variables with
project-managed default values that supports both:

- users explicitly choosing the default; and
- using the project default even as the default changes over time.

The :command:`option` command gains new keyword arguments to manage any
kind of user option.  These include ``TYPE <type>``, ``VALUES <value>..``,
``ADVANCED``, ``DEFAULT`` arguments.
