kauth: Use strl* functions in ka_log

Switch to using the strlcat and strlcpy functions in ka_log, to
avoid potential buffer overflows.

Caught by coverity (#985824)

Reviewed-on: http://gerrit.openafs.org/9397
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit b535059d48ac592760e2e5b87414d9010143c993)

Change-Id: Ie2a3f39e3189492f855729b4ded7a6fa71c43c9c
Reviewed-on: http://gerrit.openafs.org/11046
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>