hostnamed: make sure we can actually properly parse 'allowInteractiveAuthentication...

hostnamed: make sure we can actually properly parse 'allowInteractiveAuthentication' varlink parameter

If people want they should be able to turn on this flag, to allow
interactive auth. Let's make sure this actually works. i.e. add it to
the introspection data and don't refuse the parameter in Describe().
(note the varlink handling already does parameter validation through
varlink_dispatch(), hence we can just drop any further validation)