Changed how the directory and webdav special remotes store content.
The new method uses less system calls, is more robust, and leaves any
partially transferred key content in a single tmp directory, which
will make it easier to clean that out later.

Also found & fixed a cute bug in the directory special remote when the
chunksize is set to a smaller value than the ByteString chunk size, that
caused it to loop forever creating empty files.

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Added an embedcreds=yes option when setting up special remotes. 
Will put UI for it into the webapp later, but rather than work on that
tomorrow, I plan to work on glacier.