Extend _snapshot_tree to record mode, inode, xattrs, directories and
special nodes, and add coverage proving a server-contacting --dry-run
leaves the destination structurally identical for --delay-updates,
--backup, symlinks, hardlinks, FIFOs, and daemon modules (including a
read-only module). Add a regression test for the --read-batch --dry-run
refusal and for a missing/non-directory receive root failing a dry-run
exactly like a real run.
Fix stale version comments (2.20.0/633 -> 2.21.0/637) and RSYNC_COMPAT's
current --protocol value, and add a unit assertion that
--server-port/--port (and --server-host) set the dry-run routing bit.