Adds ignore_errors (client-only) and force_delete (wire) booleans, makes max_delete default -1 (no client limit), and parses --delete-excluded, --max-delete=NUM, --ignore-errors, --force, --prune-empty-dirs. Bumps PROTOCOL_VERSION 2.8.0 -> 2.9.0 for the new on-the-wire force_delete field.