Fix node handling on package addition/removal

* UnpackingLeafNode: Add a fFinalPackageNode attribute. It is set when
the node is about to be removed and will point to the node's previous
head package node and be used in its stead. From the perspective of
the FS hooks this leaves the node in an unchanged state.
* Unpacking[Leaf,Directory]Node:
- Add WillBeFirstPackageNode(), returning whether the given package
node would become the head package node when added.
- Add PrepareForRemoval() which removes all package nodes. In case of
UnpackingLeafNode it also sets fFinalPackageNode.
- Add CloneTransferPackageNodes(). It is only implemented for
UnpackingLeafNode. It clones the node, transfers all package nodes
to the clone and sets fFinalPackageNode on this node.
* Volume::_{Add,Remove}PackageNode(): Solved the following TODO: When a
package is added or removed and a file present in both the
added/removed package and another package with the version in the
former having precedence, we have to remove the node (leaving it
unchanged) and replace it by a new node. This prevents clients having
the node opened or mapped from suddenly seeing different data. It also
fixes unbalanced calls to PackageNode::VFSInit()/VFSUninit() which
would result in file descriptors to package files being leaked.

Add UnpackingNode::IsOnlyPackageNode()

It returns whether the given package node is the only package node
attached to that node.

Attribute index support for non-special attributes

* Add AttributeIndex class.
* Each attribute does now have an attribute index cookie. The new
attribute index service methods Node::IndexAttribute() and
IndexCookieForAttribute() create+set/retrieve the cookie. The cookie
is actually the attribute index's tree node.
* Add OldNodeAttribute::IndexCookieForAttribute() so the cookie is
available when the node changes.

Add Node::Init() flags parameter

* Add Node::Init() flags:
- NODE_FLAG_KEEP_NAME: Take over ownership of the name.
- NODE_FLAG_CONST_NAME: Don't copy the name -- it's a constant that
lives at least as long as the object.
* Remove Init() implementations in derived classes that just call the
base class version.

UnpackingDirectory: Remove unnessary methods

Remove Read() and ReadSymlink() methods which are already implemented in
Directory.

Pull derived UnpackingDirectory out of Directory

Move all package specifics from now abstract Directory to new derived
class UnpackingDirectory and adjust the Volume implementation
accordingly. This concludes the Node/Directory refactoring. Neither
class is aware of packages anymore.

Fix node handling on package addition/removal

* UnpackingLeafNode: Add a fFinalPackageNode attribute. It is set when
the node is about to be removed and will point to the node's previous
head package node and be used in its stead. From the perspective of
the FS hooks this leaves the node in an unchanged state.
* Unpacking[Leaf,Directory]Node:
- Add WillBeFirstPackageNode(), returning whether the given package
node would become the head package node when added.
- Add PrepareForRemoval() which removes all package nodes. In case of
UnpackingLeafNode it also sets fFinalPackageNode.
- Add CloneTransferPackageNodes(). It is only implemented for
UnpackingLeafNode. It clones the node, transfers all package nodes
to the clone and sets fFinalPackageNode on this node.
* Volume::_{Add,Remove}PackageNode(): Solved the following TODO: When a
package is added or removed and a file present in both the
added/removed package and another package with the version in the
former having precedence, we have to remove the node (leaving it
unchanged) and replace it by a new node. This prevents clients having
the node opened or mapped from suddenly seeing different data. It also
fixes unbalanced calls to PackageNode::VFSInit()/VFSUninit() which
would result in file descriptors to package files being leaked.

Add UnpackingNode::IsOnlyPackageNode()

It returns whether the given package node is the only package node
attached to that node.

Attribute index support for non-special attributes

* Add AttributeIndex class.
* Each attribute does now have an attribute index cookie. The new
attribute index service methods Node::IndexAttribute() and
IndexCookieForAttribute() create+set/retrieve the cookie. The cookie
is actually the attribute index's tree node.
* Add OldNodeAttribute::IndexCookieForAttribute() so the cookie is
available when the node changes.

Add Node::Init() flags parameter

* Add Node::Init() flags:
- NODE_FLAG_KEEP_NAME: Take over ownership of the name.
- NODE_FLAG_CONST_NAME: Don't copy the name -- it's a constant that
lives at least as long as the object.
* Remove Init() implementations in derived classes that just call the
base class version.

UnpackingDirectory: Remove unnessary methods

Remove Read() and ReadSymlink() methods which are already implemented in
Directory.

Pull derived UnpackingDirectory out of Directory

Move all package specifics from now abstract Directory to new derived
class UnpackingDirectory and adjust the Volume implementation
accordingly. This concludes the Node/Directory refactoring. Neither
class is aware of packages anymore.