Various bug fixes, and work on the OSX app today:

* Avoid crashing when ssh-keygen fails due to not being able to parse
  `authorized_keys`.. seems a lot of people have crufty unparsable
  `authorized_keys` files.
* On OSX, for some reason the webapp was failing to start sometimes due
  to bind failing with EINVAL. I don't understand why, as that should
  only happen if the socket is already bound, which it should not as
  it's just been created. I was able to work around this by retrying
  with a new socket when bind fails.
* When setting up `authorized_keys` to let `git-annex-shell` be run,
  it had been inserting a perl oneliner into it. I changed that
  to instead call a `~/.ssh/git-annex-shell` wrapper script that it sets
  up. The benefits are it no longer needs perl, and it's less ugly,
  and the standalone OSX app can modify the wrapper script to point to
  wherever it's installed today (people like to move these things around I
  guess).
* Made the standalone OSX app set up autostarting when it's first run.
* Spent rather a long time collecting the licenses of all the software that
  will be bundled with the standalone OSX app. Ended up with a file
  containing 3954 lines of legalese. Happily, all the software appears
  redistributable, and free software; even the couple of OSX system libraries
  we're bundling are licensed under the APSL.