Exports this context so that it can be imported in another
process, providing the inter-process context transfer service is
available (c.f. GSSAPI.TRANS_FLAG ).
This wraps GSS_Export_sec_context according to RFC 2743
section 2.2.8.
The returned string is intended to be fed to
GSSAPI.Context.create (or some other
GSS_Import_sec_context wrapper) in the receiving process.
This operation frees the context in this object.