Fix continuations in context establishment calls
    
    Properly support continuations, including returning the rigth error code
    and exporting partial contexts.
    
    Fixes multistep authentications in particular for the initialization case
    which always uses continuations.
    
    Resolves: https://fedorahosted.org/gss-proxy/ticket/108
    
    Reviewed-by: Günther Deschner <gdeschner@redhat.com>