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.

doc/ostree-sections.txt

@@ -386,6 +386,7 @@ ostree_sysroot_get_booted_deployment
 ostree_sysroot_get_deployment_directory
 ostree_sysroot_get_deployment_origin_path
 ostree_sysroot_cleanup
+ostree_sysroot_prepare_cleanup
 ostree_sysroot_get_repo
 ostree_sysroot_deployment_set_kargs
 ostree_sysroot_write_deployments