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/client/client_cli.c

@@ -475,6 +475,7 @@ static const OptionEntry OPTION_TABLE[] = {
     {"--copy-links", NULL, OPT_FLAG, offsetof(Config, copy_links)},
     {"--safe-links", NULL, OPT_FLAG, offsetof(Config, safe_links)},
     {"--copy-unsafe-links", NULL, OPT_FLAG, offsetof(Config, copy_unsafe_links)},
+    {"--hard-links", "-H", OPT_FLAG, offsetof(Config, preserve_hard_links)},
     {"--sparse", "-S", OPT_FLAG, offsetof(Config, preserve_sparse)},
     {"--inplace", NULL, OPT_FLAG, offsetof(Config, inplace)},
     {"--preallocate", NULL, OPT_FLAG, offsetof(Config, preallocate)},
@@ -552,6 +553,7 @@ static const NegatableOption NEGATABLE_OPTIONS[] = {
     {"copy-links", NULL, offsetof(Config, copy_links)},
     {"safe-links", NULL, offsetof(Config, safe_links)},
     {"copy-unsafe-links", NULL, offsetof(Config, copy_unsafe_links)},
+    {"hard-links", "H", offsetof(Config, preserve_hard_links)},
     {"sparse", "S", offsetof(Config, preserve_sparse)},
     {"inplace", NULL, offsetof(Config, inplace)},
     {"preallocate", NULL, offsetof(Config, preallocate)},