Add support to query the SSF of a GSS context

Add support to query the SSF of a GSS context

Cyrus SASL provides a Security Strength Factor number to assess the
relative "strength" of the negotiated mechanism, and applications
sometimes make access control decisions based on it.

Add a call that allows us to query the mechanism that established the
GSS security context to ask what is the current SSF, based on the
enctype of the session key.

ticket: 8569 (new)