mu-init: insist that --maildir is absolute

lib/mu-store.cc

@@ -89,8 +89,12 @@ struct Store::Private {
 	Config make_config(XapianDb& xapian_db, const std::string& root_maildir,
 			   Option<const Config&> conf) {
 
-		Config config{xapian_db};
+		if (!g_path_is_absolute(root_maildir.c_str()))
+			throw Error{Error::Code::File,
+					"root maildir path is not absolute ({})",
+					root_maildir};
 
+		Config config{xapian_db};
 		if (conf)
 			config.import_configurable(*conf);
 

lib/mu-store.hh

@@ -77,8 +77,8 @@ public:
 	 * Construct a store for a not-yet-existing document database
 	 *
 	 * @param path path to the database
-	 * @param root_maildir maildir to use for this store
-	 * @param config a configuration object
+	 * @param root_maildir absolute path to maildir to use for this store
+	 * @param conf a configuration object
 	 *
 	 * @return a store or an error
 	 */