rc-update: improve usage message accuracy and consistency
rc-update: improve usage message accuracy and consistency

The old message did not indicate that the runlevel argument was optional
for add and del or that it could be used with show.

Signed-off-by: Andrew Gregory <13115e38efea9b5bf08524f6765f5bb44291d47f@gmail.com>

williamh/openrc,williamh/openrc,OpenRC/openrc,dwfreed/openrc,dwfreed/openrc,dwfreed/openrc,williamh/openrc,OpenRC/openrc,OpenRC/openrc