aklog: Avoid overflows in get_afs_mountpoint

When working with the fixed length cellname buffer, use
strlcat and strlcpy rather than strcat and strcpy.

Caught by coverity (#985763)

Reviewed-on: http://gerrit.openafs.org/9445
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit 19d2683d711f95165adc16fac765bb4a31c99043)

Change-Id: I91c169380600496a8b5148fffcb1a21feb1eee29
Reviewed-on: http://gerrit.openafs.org/11059
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>