command: added command "rescan"

"rescan" is the same as "update", but it discards existing songs in
the database.

src/main.c

@@ -362,7 +362,7 @@ int main(int argc, char *argv[])
 	if (create_db) {
 		/* the database failed to load: recreate the
 		   database */
-		unsigned job = update_enqueue(NULL);
+		unsigned job = update_enqueue(NULL, true);
 		if (job == 0)
 			g_error("directory update failed");
 	}