--nuke doesn't use cache directory

[cperciva]

This is a feature (if you lose your cache directory and want to nuke a machine, you shouldn't have to run --fsck first), but it has two downsides:

1. Not using the cache directory means not locking the cache directory, so tarsnap --nuke will be interrupted if a cron job kicks in to create an archive.
2. Not using the cache directory means not updating the cache directory, so people need to run --fsck after running --nuke if they want to keep using the same keys.

I think the answer here might be to make the cache directory optional for --nuke -- use it if present, but retain the current behaviour if not. A bit of nuance is needed in case someone wants to nuke a machine while running on a different machine which has a valid cache directory; we don't want to mangle (or even lock) the wrong cache directory. The right answer might be to try a "normal" delete transaction first, and revert to the no-cache-directory nuke pathway if that fails with an out-of-sync error.

I need to think about this some more.

[cperciva]

Note: Issue #76 is the second downside I mentioned above.