1. listpeers has a deprecated `"closer": null`, which we need
to handle in the schema, while trying not to damage our
documentation too much.
2. Don't print a condition if there are no fields to print.
3. Allow a special "untyped" marker for multifundchannel which returns
arbitrary JSON in a field.
4. Allow a single field return (for 'stop').
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
