aklog: Tidy realm handling

aklog: Tidy realm handling

Assorted small changes to the way realms are handled by aklog:
  * Remove static buffers when realms are passed between functions
  * Add cleanup for all dynamically allocated strings to auth_to_cell
  * Create an extract_realm helper function to handle extracting realm
    information from principals.
  * Add a function to handle extracting realm information from decoded
    tickets

Change-Id: I320389033f4862d30774b40103e536fd5bac841b
Reviewed-on: http://gerrit.openafs.org/1323
Reviewed-by: Alistair Ferguson <alistair.ferguson@mac.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>