[krbdev.mit.edu #7458] add more strftime format strings for klist

[Tom Yu via RT]

klist ends up calling krb5_timestamp_to_sfstring() with a max length of 20, which prevents it 
from using some of the strftime() formats in some locales.  In the en_US.UTF-8 locale on recent 
versions of Ubuntu, none of the formats except the European date-order format "%d/%m/%Y %R" 
fits in the allotted space.  Relatedly, we shouldn't hard code potentially ambiguous date formats, 
so consider using "%Y-%m-%dT%H:%M:%S", "%Y%m%dT%H%M%S", or "%Y%m%d%H%M%S" as a final 
fallback.  (These are ISO 8601 date-and-time formats.)