Provide auto argument for BTRFS_DEFRAG_PATHS

[krichter722]

Like for other path specifications it'd be nice to have the option to specifcy auto as value for BTRFS_DEFRAG_PATHS and let paths considered for defragment be detected automatically.

[sten0]

Hi @krichter722,

I'm not sure about the feasibility of this, because a snapshot could exist anywhere. One would have to: for each btrfs volume, for each directory (find -type d), and then additionally do something like this for each directory https://github.com/sten0/btrfs-borg/blob/devel/btrfs-borg-functions#L175 P.S. the devel branch of that project isn't in a usable or consistent state at the moment. [edit: oh, and additionally rw snapshots need to be checked and excluded, because snapshots should never be defragmented... There's another function for that in that file]