Don't retrieve principal data for kadmin list
kadmin list and kadmin get -t used the same output display logic
as short output, which meant that they called kadm5_get_principal
for each principal. However, they then just threw that output
away since the terse format displays only the principal name.
For terse get output, instead use a separate set of functions that
just print the string version of the principal name and do not
retrieve additional information.
Signed-off-by: Love Hornquist Astrand <lha@h5l.org>