Introduce FREEZING_SELF and FREEZING_PARENT and make FREEZING OR of
the two flags. This is to prepare for full hierarchy support.
freezer_apply_date() is updated such that it can handle setting and
clearing of both flags. The two flags are also exposed to userland
via read-only files self_freezing and parent_freezing.
Other than the added cgroupfs files, this patch doesn't introduce any
behavior change.