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.

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>
(cherry picked from commit 089cd2c1039315fe400f85eec1c9f2152ea090c7)

Change-Id: I0428c8c4dc7f6b1f667e69b571be944777bcb057
Reviewed-on: http://gerrit.openafs.org/4002
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>