sysroot: Add ostree_sysroot_prepare_cleanup()

New public function works like ostree_sysroot_cleanup() EXCEPT FOR
pruning the repository.

Under the hood, add _ostree_sysroot_piecemeal_cleanup() which takes
flags to better control what files are cleaned up.  Both public cleanup
functions are now wrappers for _ostree_sysroot_piecemeal_cleanup() with
different flags.

src/libostree/ostree-sysroot-deploy.c

@@ -489,6 +489,9 @@ checkout_deployment_tree (OstreeSysroot     *sysroot,
   if (!glnx_opendirat (sysroot->sysroot_fd, osdeploy_path, TRUE, &osdeploy_dfd, error))
     goto out;
 
+  if (!glnx_shutil_rm_rf_at (osdeploy_dfd, checkout_target_name, cancellable, error))
+    goto out;
+
   if (!ostree_repo_checkout_tree_at (repo, &checkout_opts, osdeploy_dfd,
                                      checkout_target_name, csum,
                                      cancellable, error))