xdg-app changes /self to /app

[Alexander Larsson <alexl redhat com>]

This is a public announcement to anyone following xdg-app and creating
apps. I just pushed a change to make xdg-app mount the app at /app
rather than /self. The reason for this change is to make it the same
lenght as "/usr", in case someone need to do some weird rewriting to get
things to relocate. I'm not aware of any such need atm, but we may as
well change it at this point because not much is using xdg-app yet.

For now there is a /self symlink that makes old things work, but at some
point we might remove that.