Add support of OpenRC service manager

I use Gentoo and install Pihole program from tatsh-overlay overlay.

I know, that Gentoo is not supported by Pi-Hole.
I know, that ebuild from tatsh-overlay overlay is broken and Pi-Hole at Gentoo is not usable at now.

But I want to use pihole in my Gentoo system. And biggest problem, is that pihole not supports OpenRC service manager.

Please, add support of OpenRC service manager.

Some work to start pihole-FTL under OpenRC service manager may see at
github