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/server/receiver.c

@@ -154,7 +154,7 @@ int receiver_process_pending(Config* config, int file_descriptor, const Receiver
   DeleteManifest* deferred_manifest = NULL;
   while (status == STATUS_NEXT || status == STATUS_CHUNK || status == STATUS_CHECK ||
          status == STATUS_KEEPALIVE || status == STATUS_ABORT || status == STATUS_CHECK_BATCH ||
-         status == STATUS_MKDIR || status == STATUS_MANIFEST) {
+         status == STATUS_MKDIR || status == STATUS_MANIFEST || status == STATUS_HARDLINK) {
     if (status == STATUS_KEEPALIVE) {
       if (!send_status(file_descriptor, STATUS_KEEPALIVE))
         goto fail;
@@ -181,6 +181,10 @@ int receiver_process_pending(Config* config, int file_descriptor, const Receiver
       File* dir = file_receive_directory(file_descriptor);
       if (!dir || !sink->store_file(dir, sink->context))
         goto receive_error;
+    } else if (status == STATUS_HARDLINK) {
+      File* file = file_receive_hardlink(file_descriptor);
+      if (!file || !sink->store_file(file, sink->context))
+        goto receive_error;
     } else if (status == STATUS_MANIFEST) {
       DeleteManifest* manifest = receive_manifest_entries(file_descriptor);
       if (!manifest)