aklog: Protect against overflows from cmdline

aklog: Protect against overflows from cmdline

The cell, realm and path arrays are populated based on the user's
command line, and xlog_path is populated from their passwd map
entry. Protect against all of these overflowing, by making suitable
use of strlcpy and strlcat.

Caught by coverity (#985764, #985904)

Change-Id: Ia8f1816b010eb2b85b537e156de2b7983e4626ba
Reviewed-on: http://gerrit.openafs.org/9446
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>