auto-mounting degraded multi-device root partitions

I pull this for discussion on 2916680cb700a3cb22cc8b89827e9918f0add035.

Recently I gave a lecture on btrfs filesystem at our local LUG. One upcoming concern in our discussion was about not being able to auto-mount degraded volumes. So if you have a server outside in a data-center using btrfs as root partition, you would not be able to boot after a power failure which can cause a disk to fail. So I wanted to come up with a solution for that.

I tried to implement this using `btrfs filesystem show`, but afterwards 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.