add pre-pivot-cleanup hook

add pre-pivot-cleanup hook

Sometimes some hook script will need to be before the cleanup hook scripts
For example dhclient killing, nfs cleanup, etc. must not happen before kdump
because it will use their fuctionalities.

So here introduce a new hook pre-pivot-cleanup, all cleanup scripts will go there.
that means pre-pivot hook is splited to two hooks pre-pivot and pre-pivot-cleanup

Signed-off-by: Dave Young <dyoung@redhat.com>