feat: implement -R/--relative, --no-implied-dirs, --dirs/-d, --mkpath

RSYNC_COMPAT Phase-2 row M: path-list construction and destination
directory creation while preserving traversal safety.

- -R/--relative with --files-from: transmit each listed entry under its
  bare relative destination path (no source-root mirror). Files keep an
  absolute local read path plus a separate wire/dest path (File.send_path);
  manifest, incremental quick-check and change output follow the wire path,
  so --delete and --remove-source-files stay consistent. -R without
  --files-from is unchanged (full mirror).
- --no-implied-dirs: client-only, only meaningful with -R + --files-from.
  A listed file whose parent dir is not itself (or via an ancestor)
  explicitly listed cannot be placed; the run fails up front with a clear
  error. No effect otherwise.
- --dirs/-d + --old-dirs/--old-d aliases: -d <dir> transmits the source
  root as an explicit empty directory entry (STATUS_MKDIR frame); with
  --files-from listed dirs are created empty and listed files transferred,
  never descending. Works single-threaded, -m (sequential scanner in the
  -m scan thread) and chunk-serialization (per-file type marker).
  Directory entries appear in the delete manifest.
- --mkpath: new wire bool; server creates the destination root (and missing
  leading components under its authorized root) at connection start. A
  missing destination root is now rejected by default.
- Protocol bumped to 2.7.0 (mkpath wire field + STATUS_MKDIR + chunk type
  marker). All receive paths funnel through file_save_to_disk_full which
  creates directories via the secure confined mkdir engine; dir entries are
  excluded from --remove-source-files outcome acknowledgements on both ends.
- Unit coverage: CLI parse (relative/dirs aliases/mkpath/no-implied-dirs),
  config round-trip (relative + mkpath), scanner --dirs non-recursion and
  -R send_path (sequential + parallel), receiver dir-entry save.
- Integration coverage: TestRelativeFilesFrom, TestNoImpliedDirs, TestDirs,
  TestMkpath (single and -m).
- RSYNC_COMPAT: 4 rows move to Implemented (Summary 67/3/5/1/71 = 147).

src/client/usage.c

@@ -26,8 +26,17 @@ void print_usage(void) {
   printf("  --delete            Delete files on receiver not in source\n");
   printf("  --ignore-existing  Skip files that already exist on receiver\n");
   printf("  --delay-updates    Put updated files into place only at the end of transfer\n");
-  printf(
-      "  --dirs, --old-dirs, --old-d  Transfer directories without recursing (not implemented)\n");
+  printf("  --dirs, -d, --old-dirs, --old-d  Transfer the named directory entries without\n");
+  printf("                    recursing into their contents (-d <dir> mirrors the source\n");
+  printf("                    directory empty; with --files-from listed dirs are created\n");
+  printf("                    empty and listed files are transferred)\n");
+  printf("  -R, --relative    With --files-from, preserve each listed entry's relative path\n");
+  printf("                    below the destination root instead of mirroring the full\n");
+  printf("                    source path (no effect without --files-from)\n");
+  printf("  --no-implied-dirs  With -R --files-from, refuse to place a listed file whose\n");
+  printf("                    parent directory is not itself listed\n");
+  printf("  --mkpath          Create the destination root directory on the server when it\n");
+  printf("                    does not exist yet\n");
   printf("  --del               Alias for --delete-during (not implemented)\n");
   printf("  --exclude <pattern> Exclude files matching pattern\n");
   printf("  --include <pattern> Only include files matching pattern\n");