s6-rc: new package

This new package provides s6-rc, a service manager for s6-based systems,
i.e. a suite of programs that can start and stop services, both
long-running daemons and one-time initialization scripts, in the proper
order according to a dependency tree.

The host variant is provided so s6-rc-compile is available and can
be used to build the services database offline, either in a package
Makefile or a post-build script.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>