s4:kdc: Don’t prepend useless colon to MIT KDC logging messages

s4:kdc: Don’t prepend useless colon to MIT KDC logging messages

If we pass an empty string as the ‘whoami’ parameter, MIT’s logging
facilities will prepend a mysterious colon to the message. Printing
“mitkdc: ” ought at least to be more sensible, and perhaps more closely
to match our behaviour prior to commit
dd8138236bec3635c25e5b482b7a14faa0a9c36b.

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>