As reported by @Evils-Devils in Zygo/bees#123, the bees service wrapper
can fail on account of `findmnt` not being able to identify a mounted
filesystem using the default (kernel-introspection) mechanism.
Fall back to mtab and fstab-based inspection in turn should this fail.
Introduce an extent-layer (as opposed to the existing file-level) deduplication
system for btrfs. This provides a means of finding similarities within
non-identical files, when they contain identical, aligned blocks.
