Implement more virtuals in Node and Directory

* Node: Add default implementations for UserID(), GroupID(),
OpenAttributeDirectory(), and OpenAttribute().
* Directory: Add default implementations for Mode() and FileSize().
* Remove the respective hook implementations in the PackageLink*
classes.

Add install path support to packagefs

NodeListener::NodeChanged(): Provide old value

* Add interface OldNodeAttributes an instance of which is passed to
NodeListener::NodeChanged() to provide the old attribute values
(currently only modification time and file size).
* Also extend PackageLinksListener::PackageLinkNodeChanged() with a
OldNodeAttributes parameter.
* Add OldNodeAttributes implementations for PackageLinkSymlink (inner
class OldAttributes) and UnpackingNode (OldUnpackageNodeAttributes).

PackageLinkSymlink: Init fLinkPath in constructor

Move PackageLinkDirectory::Link to own file

And rename it to PackageLinkSymlink.

Implement more virtuals in Node and Directory

* Node: Add default implementations for UserID(), GroupID(),
OpenAttributeDirectory(), and OpenAttribute().
* Directory: Add default implementations for Mode() and FileSize().
* Remove the respective hook implementations in the PackageLink*
classes.

Add install path support to packagefs

NodeListener::NodeChanged(): Provide old value

* Add interface OldNodeAttributes an instance of which is passed to
NodeListener::NodeChanged() to provide the old attribute values
(currently only modification time and file size).
* Also extend PackageLinksListener::PackageLinkNodeChanged() with a
OldNodeAttributes parameter.
* Add OldNodeAttributes implementations for PackageLinkSymlink (inner
class OldAttributes) and UnpackingNode (OldUnpackageNodeAttributes).

PackageLinkSymlink: Init fLinkPath in constructor

Move PackageLinkDirectory::Link to own file

And rename it to PackageLinkSymlink.