[krbdev.mit.edu #2736] recode krb5_get_credentials to work without a ccache

[Jeffrey Altman via RT]

There is a need to be able to call krb5_get_credentials when a 
credential cache does not exist.  This is to avoid making
krb5_get_cred_via_tkt public.  

krb5_get_credentials should generate a temporary cache in this case for
use if cross realm must be used.