kauth: Don't overflow stack when building username

knfs constructs the userName by combining the clientName.name
and clientName.instance arrays, along with a dot separator. Make
sure that the userName array is big enough to hold these, and
use strlcpy and strlcat just to make sure.

Caught by coverity (#985829)

Reviewed-on: http://gerrit.openafs.org/9351
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit cc194827a841f057654f1dbe4dcb3f6de98c1c60)

Change-Id: Iec62a0e0fb830e8bfc76896733269d0511c5a8d9
Reviewed-on: http://gerrit.openafs.org/11055
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>