Handle logging of personal zephyrs from different instances of Kerberos principals sanely

[zhangc@mit.edu]

Personal zephyrs are logged by sender/recipient, but logging appears to also differentiate by instance, so zephyrs to/from null instances are logged in a separate file from root instances or extra instances, which is annoying when trying to piece together a conversation with someone's root or extra instance, because you only ever send to their null instance, so the conversation gets chopped up between the files.