add a way to suspend the subiquity UI and run some other command
add a way to suspend the subiquity UI and run some other command

This will be used for the drop to shell functionality and also to allow
viewing an error report before we submit it to daisy.

It is almost but not quite surprisingly easy (I added a long-ish comment
about the difficulties).

CanonicalLtd/subiquity,CanonicalLtd/subiquity