Incremental sync: --incremental flag to skip unchanged files

- New STATUS_CHECK protocol status (value 6)
- Client sends path + size + mtime; server replies OK (skip) or NEXT (send)
- config.h/c: use_incremental field sent/received over wire
- file.c/h: file_send_single_calls_no_path helper for incremental path
- client_cli.c: --incremental flag
- client_send.c: per-file check before send in both single and chunk paths
- server.c: STATUS_CHECK handling in receive_files()
- multiprocessing.c: STATUS_CHECK handling in receive_thread()
- test.py: incremental sync test case

src/shared/file.h

@@ -25,6 +25,7 @@ void file_destroy(void *item);
 bool file_load_data(File *file);
 File *file_receive(Config *config, int file_descriptor);
 bool file_send_single_calls(File *file, int file_descriptor, bool use_metadata, int compression_level);
+bool file_send_single_calls_no_path(File *file, int file_descriptor, bool use_metadata, int compression_level);
 bool file_send_sendfile(File *file, int file_descriptor, bool use_metadata);
 size_t file_content_to_buffer(File *file);
 FileMetadata *file_metadata_create(struct stat *stats);