Fix bug in addprinc: use -pw option

addprinc was using the -w option of kadmin.local instead of the -pw option of addprinc. The former is for authenicating access to kadmin.local (not necessary for root/admin@REALM, and therefore ignored), whereas the later is for setting the password of the new principal, which is what was meant.

This patch also makes addprinc require that the krb5-kdc service be running, as this is necessary for this command.