If credentials changed try to store them
    
    This allows for efficient caching when new tickets are added on the proxy side
    when using memory ccaches.  It also allows us to store changes in flags and
    other ccache ancillary modifications.
    
    Fix impersonate tests to cope with the acquire_cred behavior change.
    
    Signed-off-by: Simo Sorce <simo@redhat.com>
    [rharwood@redhat.com: fix induced merge conflict, hoist gssx_cred, tweak
    commit message body, fix extra argument to DEBUG in t_impersonate.c]
    Reviewed-by: Robbie Harwood <rharwood@redhat.com>