MHU: Most of GHCi commands do not work

The interface for GHCi heavily assumes that there is only one home unit in a session.

In particular any commands which mutates the session state are currently disabled.

For example, :set, :add, :load etc are all disabled.

In order to close this ticket it would be necessary to reconsider how the API of GHCi works and generalise it to work with multiple home units.