feat: add -f/--sendfile for zero-copy file transfer

- Add file_send_sendfile() using sendfile() syscall to send file
  content directly from fd to socket, bypassing userspace memory
- Add use_sendfile field to Config struct (default false)
- Add -f / --sendfile flag parsing in client.c
- sendfile path in send_chunk() used when -f is set without -c or -s
- send_files_multithreaded() falls back to single-threaded when
  sendfile is enabled (loader pipeline becomes unnecessary)
- Add Sendfile (-f) test case to test.py

src/shared/config.c

@@ -20,6 +20,7 @@ Config *config_create(char *version, char *send_directory,
   config->use_compression = use_compression;
   config->compression_level = compression_level;
   config->num_connections = num_connections;
+  config->use_sendfile = false;
   return config;
 }