butc: -Wformat-truncation fallout

butc: -Wformat-truncation fallout

Increase some buffer sizes to appease gcc8.  While here, use snprintf
instead of plain sprintf(!).

Change-Id: I39d29522b92070ce2845ba3d392aaf2d97fc7b6e
Reviewed-on: https://gerrit.openafs.org/13468
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>