auth: Don't overflow cell string

If the kernel gives us bogus data back from the VIOCGETTOK pioctl,
we might overflow the cell string when copying in to it. Use
strlcpy to avoid this (unlikely) occurrence.

Caught by coverity (#985768, #985769)

Change-Id: I2583b017e7a366f4271f356216bdd60f3a7b7911
Reviewed-on: http://gerrit.openafs.org/9349
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>