Use new GSSAPI Credential Store API
    
    This is the only thread safe way to pass in aribitrary values for all the bits
    of environment we want to use when doing impersonation within gss-proxy.
    
    Requires MIT version 1.12 for the client_keytab part to be operational.
    
    Signed-off-by: Simo Sorce <simo@redhat.com>
    Reviewed-by: Günther Deschner <gdeschner@redhat.com>