feat: add -x/--one-file-system to stay within the source filesystem

Capture the transfer root's device (st_dev) at scanner creation and skip
descending into any subdirectory on a different device (a mount point).
Implemented sender/client-side only: sequential BFS and parallel (-m) root
scan apply the same scanner_same_filesystem decision; no wire/protocol change
and default behavior is unchanged. Unit tests cover the pure decision, same
device scanning in both modes, and CLI parsing; integration tests prove -x
leaves a single-filesystem tree byte-identical and, when root can mount a
tmpfs, skips a genuine cross-device subtree.

src/shared/config.c

@@ -93,6 +93,7 @@ static void config_set_defaults(Config* config) {
   config->files_from = NULL;
   config->cvs_exclude = false;
   config->prune_empty_dirs = false;
+  config->one_file_system = false;
   config->relative = false;
   config->rsh_command = NULL;
   config->rsync_path = NULL;