auth: Don't overflow buffer in CompFindUser

The fullname buffer in CompFindUser is theoretically big enough
to take the data usually supplied to it. However, play it safe by
using strlcat and strlcpy to catch buffer overflows.

Caught by coverity (#985771)

Reviewed-on: http://gerrit.openafs.org/9543
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit cc95fca8e16f83d7dda3f09a5133dc9294299d61)

Change-Id: I900611e13d6254c4410915b0688b18a3b6c4acc3
Reviewed-on: http://gerrit.openafs.org/11063
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>