Add utility function to compare gssx_creds
    
    This is not a full comparison, just enough to detect if creds have
    changed in an acquire_cred loop to a proxy.
    
    Signed-off-by: Simo Sorce <simo@redhat.com>
    [rharwood@redhat.com: style fixups, language in comment]
    Reviewed-by: Robbie Harwood <rharwood@redhat.com>