Fix stacked runlevel support

Patch was provided by  Max Hacking <max.gentoo.bugzilla@hacking.co.uk>
and slightly fixed by Alexander Vershilov <qnikst@gentoo.org> and
William Hubbs <williamh@gentoo.org>.

Fixes:
1). Rebase to newest OpenRC version.
2). Remove code style fixes. Port to currect code style.
3). Fix rc_runlevel_stack instead of introducing new function.
4). Make get_runlevel_chain a private function.

X-Gentoo-Bug: 467368
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=467368