scripts/kernelng: short help consistency

Consistently use periods for all the short help (required if we are to
rely on the automatic short-help-from-long-help convention
nontrivially).

Also provide explicit short_help values in instances where the short
help would otherwise be cropped past the apparently non-configurable
auto-short-help-cropping limit of 45 characters (grrrr...)

Signed-off-by: Gregory M. Turner <gmt@be-evil.net>