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.

Resolve dependencies to resolvables

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.

Add package link directories to system volume

* Add PackageLinksListener interface. It is used by
PackageLink[s]Directory to notify a listener about changes.
* Volume does now implement PackageLinksListener and recursively adds/
removes package link directories.

This makes the package link directories appear at least. They are still
empty.

WIP: Create empty package links directories

* Rename PackageFamily to PackageLinkDirectory and derive from
Directory.
* PackageLinksDirectory: Add the PackageLinkDirectory objects as
children and remove the no longer needed hash table (we use the
Directory's child hash table).

Move PackageFamily table to PackageLinksDirectory

Add yet empty package-links directory

* Add PackageLinksDirectory Directory subclass. Currently not doing
anything.
* PackageFSRoot: Create a PackageLinksDirectory.
* Volume: Add/remove the package links directory for the system volume.

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.

Resolve dependencies to resolvables

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.

Add package link directories to system volume

* Add PackageLinksListener interface. It is used by
PackageLink[s]Directory to notify a listener about changes.
* Volume does now implement PackageLinksListener and recursively adds/
removes package link directories.

This makes the package link directories appear at least. They are still
empty.

WIP: Create empty package links directories

* Rename PackageFamily to PackageLinkDirectory and derive from
Directory.
* PackageLinksDirectory: Add the PackageLinkDirectory objects as
children and remove the no longer needed hash table (we use the
Directory's child hash table).

Move PackageFamily table to PackageLinksDirectory

Add yet empty package-links directory

* Add PackageLinksDirectory Directory subclass. Currently not doing
anything.
* PackageFSRoot: Create a PackageLinksDirectory.
* Volume: Add/remove the package links directory for the system volume.