auth: Don't overflow lower case cell string

When building tcell_l in kerberosSuperUser, make sure that we
don't overflow the string that we're constructing.

Use the opr_lcstring function to do the lower case conversion,
rather than rolling our own.

Caught by coverity (#985772)

Change-Id: I6e28cfc54883aac7e3a3eb2f4e2b2bf7ebc0bc63
Reviewed-on: http://gerrit.openafs.org/9544
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>