Builds on x86_64.
 SHOULD build on all other supported architectures.

 - Better s6 layout, with a place for early services and a
place for packages to add their services later on.
 - s6-svscan is now supervised by sysvinit (instead of being
run once by openrc)
 - s6-svscan is now the only process supervised by sysvinit.
All the other "respawn" lines are delegated to s6.
 - utmpd and wtmpd are now early services instead of being
added by openrc.

 These changes implement a full supervision architecture and
make init more flexible. Later on, it will be easier to
 - add conditional gettys (for /dev/hvc0...)
 - remove sysvinit entirely
 - decouple the init process from the service manager.

This way, -posix and -lsb are suitable root packages.