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).

tests/test_config.c

@@ -126,6 +126,7 @@ static void test_config_send_receive() {
   send_cfg->use_compression = true;
   send_cfg->use_metadata = true;
   send_cfg->use_executability = true;
+  send_cfg->preserve_hard_links = true;
   send_cfg->use_delta = true;
   send_cfg->whole_file = true;
   send_cfg->fuzzy = true;
@@ -181,6 +182,8 @@ static void test_config_send_receive() {
         ok = false;
       if (!recv_cfg->use_executability)
         ok = false;
+      if (!recv_cfg->preserve_hard_links)
+        ok = false;
       if (!recv_cfg->size_only)
         ok = false;
       if (!recv_cfg->ignore_times)