aklog: Avoid overflows in get_afs_mountpoint

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)

Change-Id: Idfb3a0562d4028f5d1aa134b7ab0b5fa2dd60edb
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>