Handling 'nodatacow' volumes on Btrfs

[tasket]

Volumes requested for backup might have +C nodatacow flag set, or may be in a mounted fs with nodatacow. Probably this would result in an error with current Wyng, if specified as normal volumes.

Possible ways to support this:

• Treat the volume like an import; there will be no acceleration from snapshots.
• Require the user manually specify such volumes as imports; only docs need to be changed.

Not likely:

• Use a snapshot of the subvolume; useful only if user wants to backup a hot volume (although the user could set this up themselves and tell Wyng to backup the snapshot path).