butc: -Wformat-truncation fallout

butc: -Wformat-truncation fallout

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

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>
(cherry picked from commit a89297a066d8689f8fc29a7428cfe3ed6235d010)

Change-Id: If26312359d62649077e1f8adf2c9e0b39d28496f
Reviewed-on: https://gerrit.openafs.org/13737
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Marcio Brito Barbosa <mbarbosa@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>