Finished up the automatic recovery from stale lock files. Turns out git
has quite a few lock files; the assistant handles them all.

Improved URL and WORM keys so the filenames used for them
will always work on FAT (which has a crazy assortmeny of illegal
characters). This is a tricky thing to deal with without breaking backwards
compatability, so it's only dealt with when creating new URL or WORM keys.

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I think my next step in this disaster recovery themed month will be adding
periodic incremental fsck to the assistant. `git annex fsck` can already
do an incremental fsck, so this should mostly involve adding a user
interface to the webapp to configure when it should fsck. For example, you
might choose to run it for up 1 hour every night, with a goal of checking
all your files once per month. Also will need to make the assistant do
something useful when fsck finds a bad file (ie, queue a re-download).