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)

Change-Id: Icb537567f8ae67ecb42332cda4413274edeaa681
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>