This integrates them with configvars properly: they almost "just work"
in listconfigs now, and we don't put them in a special sub-object
under their plugin.
Unfortunately, this means `listconfigs` now has a loose schema: any
plugin can add something to it.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Fixed: Plugins: reloaded plugins get passed any vars from configuration files.
Changelog-Deprecated: Config: boolean plugin options set to `1` or `0` (use `true` and `false` like non-plugin options).