Add an export builtin, and API to write to libarchive

At the moment I'm looking at using rpm-ostree to manage RPM inputs
which can then be converted into Docker images.  It's most convenient
if we can stream directly out of libostree rather than doing a
checkout + tar combination.

There are also backup/debugging etc. reasons to implement `export` as
well.

src/libostree/ostree-repo.h

@@ -441,6 +441,23 @@ gboolean      ostree_repo_write_dfd_to_mtree (OstreeRepo                 *self,
                                               GCancellable               *cancellable,
                                               GError                    **error);
 
+/**
+ * OstreeRepoWriteArchiveOptions:
+ *
+ * An extensible options structure controlling archive creation.  Ensure that
+ * you have entirely zeroed the structure, then set just the desired
+ * options.  This is used by ostree_repo_write_tree_to_archive().
+ */
+typedef struct {
+  guint disable_xattrs : 1;
+  guint reserved : 31;
+
+  guint64 timestamp_secs;
+
+  guint unused_uint[8];
+  gpointer unused_ptrs[8];
+} OstreeRepoArchiveOptions;
+
 gboolean      ostree_repo_write_archive_to_mtree (OstreeRepo                   *self,
                                                   GFile                        *archive,
                                                   OstreeMutableTree            *mtree,
@@ -449,6 +466,13 @@ gboolean      ostree_repo_write_archive_to_mtree (OstreeRepo                   *
                                                   GCancellable                 *cancellable,
                                                   GError                      **error);
 
+gboolean ostree_repo_write_tree_to_archive (OstreeRepo                *self,
+                                            OstreeRepoArchiveOptions  *opts,
+                                            OstreeRepoFile            *root,
+                                            void                      *archive,  /* Really struct archive * */
+                                            GCancellable             *cancellable,
+                                            GError                  **error);
+
 gboolean      ostree_repo_write_mtree (OstreeRepo         *self,
                                        OstreeMutableTree  *mtree,
                                        GFile             **out_file,