CPP11 — Add _narrow() operation to each POA servant

Summary: 1. The C++ spec requires that all POA servant classes derive from
PortableServer::ServantBase as a virtual base class. For C++
environments without RTTI, it is then impossible for the programmer to
narrow a servant received from the POA via reference_to_servant() or
id_to_servant() back to the programmer"s specific servant class.

Proposal: Add a _narrow() operation to each POA servant class in the
same fashion that _narrow() operations are defined for exception
classes:

// IDL
interface A { };

// C++
class POA_A : public virtual PortableServer::ServantBase

;

The _narrow() operation will return a valid pointer if the servant isa
POA_A, otherwise it returns 0.

:ServantBase as a virtual base class. For C++