sysroot: Add ostree_sysroot_prepare_cleanup()

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.