Follow-up to c9fe9afa, proper fix for HOOK_BASFFS_UP mess

Instead of the revert in c9fe9afa, we restore HOOK_BASFFS_UP to its
proper place at the end of fs_mount_all().  For this to not cause any
regressions we add a new hook, HOOK_SVC_PLUGIN, and update all plugins
that call service_register() to run at the new hook.

This will cause regressions for external 3rd party plugins that rely on
HOOK_BASEFS_UP to be called at its previous postion.  Nevertheless, this
is the proper fix to the problem.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

plugins/modules-load.c

@@ -212,10 +212,8 @@ static void load(void *arg)
 }
 
 static plugin_t plugin = {
-	.name = __FILE__,
-	.hook[HOOK_BASEFS_UP] = {
-		.cb  = load
-	},
+	.name                  = __FILE__,
+	.hook[HOOK_SVC_PLUGIN] = { .cb  = load },
 };
 
 PLUGIN_INIT(plugin_init)