New callback added to the vtable.

This is for authorising setting default contect.
Some objects wrap other objects are not required to explicitly set the
default context.
Default context unauthorise callback method implemented.
Default context set for the ztrows. However sinze ztrow class
does not authorise this therefore it shall not be effected.
unauthorise callback method implemented.

Returns ok unless modified by child.
Header file formatted.
Test program updated.
Reference counter of device object implementation corrected.