Changes

SGSN: BSSGP_ConnHdlr: GMM Service Request: add support to expect authentication

On a Service Request, the authentication is optional.
Either an authentication must happen or the key material from the previous authentication
has to be used.
The default behavior is still the same.

Change-Id: I9ed08160bf5413e3a30d6e90eb947d4d799e1058