lightningd: remove deprecated_apis global, put into lightningd.

We usually have access to `ld`, so avoid the global.

The only place generic code needs it is for the json command struct,
and that already has accessors: add one for libplugin and lightningd
to tell it if deprecated apis are OK.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

lightningd/plugin.h

@@ -130,6 +130,7 @@ struct plugins {
  * command line and passing them off to the plugin
  */
 struct plugin_opt {
+	struct plugin *plugin;
 	/* off plugin->plugin_opts */
 	struct list_node list;
 	/* includes -- prefix! */