Mention KRB5CCNAME in the aklog man page

Mention KRB5CCNAME in the aklog man page

AFS users not otherwise familiar with Kerberos may not realize that
one sets KRB5CCNAME to use an alternative ticket cache.  Mention the
variable in the aklog man page, although defer to the Kerberos
documentation for most details.

Change-Id: I13133285f153f86e0aa1e95b24715e01d576075d
Reviewed-on: http://gerrit.openafs.org/2761
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Rod Widdowson <rdw@steadingsoftware.com>
Reviewed-by: Russ Allbery <rra@stanford.edu>