plugin: Add hooks that a plugin might register

This is the first use of the `hooks` autodata field, and it required a
dummy element in order for the section not to be dropped, it'll be
removed once we have actual hooks.

lightningd/plugin_hook.c

@@ -1,5 +1,35 @@
 #include <lightningd/plugin_hook.h>
 
+static struct plugin_hook *plugin_hook_by_name(const char *name)
+{
+	static struct plugin_hook **hooks = NULL;
+	static size_t num_hooks;
+	if (!hooks)
+		hooks = autodata_get(hooks, &num_hooks);
+
+	for (size_t i=0; i<num_hooks; i++)
+		if (streq(hooks[i]->name, name))
+			return hooks[i];
+	return NULL;
+}
+
+bool plugin_hook_register(struct plugin *plugin, const char *method)
+{
+	struct plugin_hook *hook = plugin_hook_by_name(method);
+	if (!hook) {
+		/* No such hook name registered */
+		return false;
+	} else if (hook->plugin != NULL) {
+		/* Another plugin already registered for this name */
+		return false;
+	}
+	hook->plugin = plugin;
+	return true;
+}
+
+/* FIXME(cdecker): Remove dummy hook, once we have a real one */
+REGISTER_PLUGIN_HOOK(hello, NULL, void *, NULL, void *, NULL, void *);
+
 void plugin_hook_call_(struct lightningd *ld, const struct plugin_hook *hook,
 		       void *payload, void *cb_arg)
 {