Implement gss_set_cred_option()

[simo5]

Although libkrb5 should be able to handle a mechanism w/o this function implemented, it seem that, at least in some version, lack of the function causes a series of events that will end up with libkrb5 trying to dereference a gssntlmssp context as a union context and crashes.

Implementing, even an empty function would prevent the issue.