Interactively offers the user to set up the default root path.
Default root path to set.
If the cache root path is already set, it is used and nothing is done.
If the "default" root path (
defaultPath) exists, it is used,
otherwise, if running interactively, the user is asked to approve
the usage (and creation) of the default root path.
In all other cases, the cache root path is set to a session-specific
Returns (invisibly) the root path,
NULL if running a non-interactive session.