feat: basis-dir config/CLI for --compare-dest/--copy-dest/--link-dest

Replace the vestigial single compare_dest/copy_dest/link_dest Config fields with
an ordered BasisDest list (type + path per entry) that is serialized to the
receiver and interpreted relative to the destination root.  Paths must be
relative with no '.'/'..' components (confined like --backup-dir); trailing
slashes are normalized.  Wire layout changes, so PROTOCOL_VERSION -> 2.8.0.

CLI: each flag is parsed in both --flag=DIR and --flag DIR forms, is
repeatable, and keeps command-line order as basis priority.  Supplying any
basis dir implies --incremental (and therefore metadata) on the sender because
the unchanged decision is receiver-side; combining basis dirs with -s chunk
serialization is rejected in validate_config.  Usage text updated.

src/client/client_validation.c

@@ -11,6 +11,12 @@ bool validate_config(const Config* config) {
     print_usage();
     return false;
   }
+  if (config_has_basis(config) && config->use_chunk_serialization) {
+    log_message(LOG_LEVEL_ERROR,
+                "--compare-dest/--copy-dest/--link-dest require per-file incremental checks and "
+                "cannot be combined with -s (chunk serialization)");
+    return false;
+  }
   if (config->use_sendfile && (config->use_chunk_serialization || config->use_compression)) {
     log_message(LOG_LEVEL_ERROR, "-f/--sendfile cannot be combined with -c (compression) or -s "
                                  "(chunk serialization)");