Fix typo that made change detection break

We were comparing the old and old mtimes rather than the old and
the new. This meant we missed some file changes where only the mtime
changed.
Merge pull request #2330 from alexlarsson/dm-fix-change-detection

Fix typo that made change detection break
Partial merge of devmapper/ in order to integrate it as a backend
plugin.

The merge is inert, in other words the devmapper code is not called
and the primary aufs backend is untouched.
Re-incorporate ChangesDirs() for the use of the devmapper backend,
without changing existing prototypes.

This is a neutral change. The primary aufs backend is still in place
and not affected.
archive.ApplyLayer(): parse a tar archive as a standard aufs-compatible diff, and apply it on a directory
Work In Progress: devmapper backend plugin
Work In Progress: fix devmapper plugin, first independent test passes.
devmapper: rename Create() to OnCreate(), cleanup tests
devmapper: implement OnRemove
devmapper: renamed DMBackend to Driver
devmapper: rename plugin*.go to driver*.go
Integrate devmapper and aufs into the common "graphdriver" framework.

aufs is still enabled by default, no mechanism for switching drivers
yet.
Update Dockerfile to consistently use tabs as the command argument separator
Add libdevmapper static compilation to Dockerfile for device-mapper
Add graph driver registration
Merge pull request #1 from crosbymichael/register-plugins

Add graph driver registration
Merge branch 'dm-plugin-dockerfile' into 0.6.5-dm-plugin
Finalize driver interfaces
Merge pull request #2 from tianon/dm-plugin-dockerfile

Hack: Add lvm2 static compilation to Dockerfile
Merge branch 'finalize-interfaces' into 0.6.5-dm-plugin