rc: add usage message

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

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

Remind users that if they put a long-running process in /etc/local.d,
they can delay their boot or shutdown processing.

Reported-by: toralf.foerster@gmx.de
X-Gentoo-Bug: 456650
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=456650