Before this change there was lots of duplicated code in all the
dircache using backends to support DirMove.
This change factors this code into the dircache library.
Dircache was changed to:
- Remove special cases for the root directory
- Remove Fatal errors
- Call FindRoot on behalf of the user wherever possible
- Bring up to modern Go standards
Backends were changed to:
- Remove calls to FindRoot
- Change calls to FindRootAndPath to FindPath
- Don't make special cases for the root
This fixes several corner cases, for example removing a non existent
directory if FindRoot hasn't been called.
This adds expire and unlink fields to the PublicLink interface.
This fixes up the affected backends and removes unlink parameters
where they are present.
