Only re-initialize backup state if @pm@ metadata is missing,

to defensively work around a still-mysterious bug where the
list of saved packages ends up being empty even though we still
have state pending.  If we do re-initialize, then wipe all state
to make sure the right thing happens.

Don't keep open journal files -- close them after every update.
A bit less efficient, but possibly more reliable (again, this is
defensive programming here).  Also change "rwd" to "rws" mode
for fully synchronous operation.