hard-links: -H/--hard-links preserves inode relationships

Source files sharing (st_dev,st_ino) are recreated as hard links on the
destination; only the first member's data crosses the wire (siblings ride a
payload-less STATUS_HARDLINK frame). Ordering requires the single-FIFO-writer
receiver + forced sequential scan (documented). link()-failure falls back to a
byte-identical local copy. Rejects -s/--append. PROTOCOL_VERSION 2.11.0->2.12.0.
Review fixes: delete the dead HardLinkRegistry (ordering holds by FIFO writer),
and --existing no longer aborts when the first member is absent but the sibling
exists (leaves the sibling in place).

src/shared/config.c

@@ -195,6 +195,8 @@ static bool validate_received_config(const Config* config) {
             which chunk serialization -s disables: reject on the receiver too
             so a -s sender cannot negotiate an inert append mode. */
          !((config->append || config->append_verify) && config->use_chunk_serialization) &&
+         !(config->preserve_hard_links && config->use_chunk_serialization) &&
+         !(config->preserve_hard_links && (config->append || config->append_verify)) &&
          (!config->use_compression ||
           (config->compression_level >= 1 && config->compression_level <= 22)) &&
          config->chunk_size > 0 && config->chunk_size <= MAX_CHUNK_SIZE &&