auto-mounting degraded multi-device root partitions

I pull this for discussion on 2916680cb700a3cb22cc8b89827e9918f0add035.

I tried to implement the same using `btrfs filesystem show`, but then I thought it would be better to not rely on the output of btrfs commands, because they are still in development. Also there is no additional benefit using this method, beside preventing the first unsuccessful mount and its error output.

In my opinion we need to get the explicit approval from a user to mount degraded volumes automatically, because it may do additional harm to the filesystem. That's why I implemented it disabled by default.