(#13880) Add openrc service provider for Gentoo and Funtoo

Gentoo uses openrc (at least since baselayout-2) to
start/stop/enable/disable services. This can be done with the following
commands

/bin/rc-status   - show runlevels
/sbin/rc-service - start/stop a service
/sbin/rc-update  - enable/disable a service

The openrc provider was written as a replacement for the current gentoo
provider. The gentoo provider does just use the openrc commands to
enable/disable a service but it still calls the initscripts directly to
start a stop service. The gentoo provider does also have problems with
long service names (#5197).

The reason why this is a seperate provider: #5197 states that the
rc-service command was probably introduced when gentoo upgraded from
baselayout-1 to baselayout-2 (I wasn't really able to verify this)
so the openrc provider will not work on all gentoo systems.

It is also notable that the new openrc provider does not put any confines
on operatingsystem so the service provider may also be used on other
systems that use openrc (like the Gentoo fork "Funtoo")