lib-storage: Add struct mail_user.namespaces_created

This makes it easier to determine in mail_namespaces_added hook if the
namespaces being added are the initial ones or later ones.

src/lib-storage/mail-namespace.c

@@ -388,6 +388,7 @@ int mail_namespaces_init_finish(struct mail_namespace *namespaces,
 		*error_r = t_strdup(namespaces->user->error);
 		return -1;
 	}
+	namespaces->user->namespaces_created = TRUE;
 	return 0;
 }
 

src/lib-storage/mail-user.h

@@ -72,6 +72,9 @@ struct mail_user {
 	unsigned int autocreated:1;
 	/* mail_user_init() has been called */
 	unsigned int initialized:1;
+	/* The initial namespaces have been created and
+	   hook_mail_namespaces_created() has been called. */
+	unsigned int namespaces_created:1;
 	/* SET_STR_VARS in user's all settings have been expanded.
 	   This happens near the beginning of the user initialization,
 	   so this is rarely needed to be checked. */