librpc/rpc: add dcerpc_binding_[g|s]et_abstract_syntax()

librpc/rpc: add dcerpc_binding_[g|s]et_abstract_syntax()

For now this is just an alternate wrapper to
access binding->object.

Currently callers are reusing binding->object to store the
abstract syntax id instead of just the object uuid.

Some services on Windows use the same GUID for the
'object' and the 'abstract syntax', but they are completely
different things!

Most services use a null object guid, some reuse the
guid of the abstract syntax and use it like a 'class' object.

But the object guid is only really used by DCOM,
in order to call functions relative to an object instance,
which a dynamically allocated object guid.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Guenther Deschner <gd@samba.org>