add explicit usage message to rc.c

Signed-off-by: Andrew Gregory <andrew.gregory.8@gmail.com>
improve rc-update usage 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 <andrew.gregory.8@gmail.com>
improve rc-status usage accuracy and consistency

This makes it more clear that many of the options are used as an
alternative to a runlevel.

Signed-off-by: Andrew Gregory <andrew.gregory.8@gmail.com>
add explicit rc-service usage message

Signed-off-by: Andrew Gregory <andrew.gregory.8@gmail.com>