static-delta: Add show subcommand

Right now though, almost all of the details of deltas are private, so
we can't do the "honest thing" and have the command line just use the
shared library.

Eventually some of this should appear in the API, but for now add
command line which is useful for debugging.

src/libotutil/ot-variant-utils.h

@@ -48,6 +48,13 @@ gboolean ot_util_variant_map (GFile *src,
                               GVariant **out_variant,
                               GError  **error);
 
+gboolean ot_util_variant_map_at (int dfd,
+                                 const char *path,
+                                 const GVariantType *type,
+                                 gboolean trusted,
+                                 GVariant **out_variant,
+                                 GError  **error);
+
 gboolean ot_util_variant_map_fd (int                  fd,
                                  goffset              offset,
                                  const GVariantType  *type,