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_receive.h

@@ -9,6 +9,7 @@
 
 File* file_receive(const Config* config, int file_descriptor);
 File* file_receive_directory(int file_descriptor);
+File* file_receive_hardlink(int file_descriptor);
 File* receive_incremental_check(int fd, const Config* config, bool* skipped);
 
 /* A received delete-manifest frame: the keep-set (`keeps`, destination-relative