libplugin: handle check for our commands.

We don't thoroughly handle `check setconfig`: it would be good to
allow this to do further checking!

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

plugins/libplugin.h

@@ -53,6 +53,7 @@ struct command {
 	const char *id;
 	const char *methodname;
 	bool usage_only;
+	bool check;
 	struct plugin *plugin;
 	/* Optional output field filter. */
 	struct json_filter *filter;