Add repository "summary" file and metalink support

For Fedora and potentially other distributions which use globally
distributed mirrors, metalink is a popular solution to redirect
clients to a dynamic set of mirrors.

In order to make metalink work though, it needs *one* file which can
be checksummed.  (Well, potentially we could explode all refs into the
metalink.xml, but that would be a lot more invasive, and a bit weird
as we'd end up checksumming the checksum file).

This commit adds a new command:

$ ostree summary -u

To regenerate the summary file.  Can only be run by one process at a
time.

After that's done, the metalink can be generated based on it, and the
client fetch code will parse and load it.

https://bugzilla.gnome.org/show_bug.cgi?id=729585

src/libotutil/ot-variant-utils.h

@@ -31,6 +31,8 @@ GVariant *ot_gvariant_new_bytearray (const guchar   *data,
 
 GVariant *ot_gvariant_new_ay_bytes (GBytes *bytes);
 
+GVariant *ot_gvariant_new_empty_string_dict (void);
+
 GHashTable *ot_util_variant_asv_to_hash_table (GVariant *variant);
 
 GVariant * ot_util_variant_take_ref (GVariant *variant);
@@ -70,5 +72,10 @@ ot_variant_new_from_bytes (const GVariantType  *type,
                            GBytes        *bytes,
                            gboolean       trusted);
 
+gboolean
+ot_variant_bsearch_str (GVariant   *array,
+                        const char *str,
+                        int        *out_pos);
+
 G_END_DECLS