It's mostly working now. Still need to fix --fast and --relaxed, and avoid
youtube-dl running out of the annex.diskreserve.

The first hour or two was spent adding support for per-key temp
directories. youtube-dl is run inside such a directory, to let it write
whatever files it needs. Like the per-key temp files, these temp directories
are not cleaned up when a download fails or is interrupted, so resuming can
pick up where it left off. Taught `git annex dropunused` and everything
else that cleans up per-key temp files to also clean up the temp
directories.

Today's work was sponsored by Trenton Cronholm on
[Patreon](https://patreon.com/joeyh/)
