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/file.c

@@ -108,6 +108,9 @@ File* file_create(const char* path) {
   file->skip = false;
   file->is_dir = false;
   file->basis_link = NULL;
+  file->link_group = 0;
+  file->link_first = false;
+  file->hardlink_target = NULL;
   return file;
 }
 
@@ -125,6 +128,8 @@ void file_destroy(void* item) {
   file->send_path = NULL;
   free(file->basis_link);
   file->basis_link = NULL;
+  free(file->hardlink_target);
+  file->hardlink_target = NULL;
   free(file);
 }