rc: allow switching runlevels without stopping services

OpenRC, by default, stops all services that are not listed in a runlevel
when rc is used to switch runlevels. This adds a -n/--no-stop command
line option to rc which tells it to skip stopping the services which are
not in the runlevel.

Reported-by: gentoo@thoth.purplefrog.com
X-Gentoo-Bug: 372585
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=372585