Kernel interface abstraction for attribute access

* Introduce interface AttributeCookie and currently only implementation
  UnpackingAttributeCookie. This is an interface for reading/stat()ing
  an attribute.
* Add abstract virtual Node::OpenAttribute() method that returns an
  AttributeCookie and implemented it for derived classes.
* In the kernel interface attribute hooks use AttributeCookie now. The
  attribute directory hooks are unchanged.

src/add-ons/kernel/file_systems/packagefs/Jamfile

@@ -7,6 +7,7 @@ UsePrivateHeaders shared ;
 
 
 HAIKU_PACKAGE_FS_SOURCES =
+	AttributeCookie.cpp
 	BlockBufferCacheKernel.cpp
 	DebugSupport.cpp
 	Dependency.cpp
@@ -26,6 +27,7 @@ HAIKU_PACKAGE_FS_SOURCES =
 	PackageNodeAttribute.cpp
 	PackageSymlink.cpp
 	Resolvable.cpp
+	UnpackingAttributeCookie.cpp
 	Version.cpp
 	Volume.cpp
 ;