Removed the inittab file and made device manager configurable via
/etc/rc.conf. The /etc/rc* stuff will become the generic interface
to the boot process (removing the busybox specific stuff and making
it easier to configure things).

Existing users:

    inittab file remains functional as it always was but no updates
    will be made to the file by the distribution going forward. The
    busybox package will still provide what it does.

    If using a device manager other than mdev (for the init process)
    you must set the correct value in /etc/rc.conf. Valid values
    are currently 'mdevd', 'mdev' (default) and 'udevd'). Open an
    issue for additional device manager support.

New users:

    New systems will not use /etc/inittab. What this means is that
    baseinit must provide configuration for starting busybox runit
    (and other service managers) with an option/fallback to starting
    some gettys for you (no service manager).

Ample notice will be given before this change goes live. There are
other changes to make too (including documentation).

Everything is still subject to change. Just moving things forward.
Right now this allows easy swap between inits (busybox init, s6,
sinit and runit) and easy swap between service managers (busybox
runit, runit, s6, none or none-getty) via /etc/rc.conf. I am still
expanding this to support other options.

The "poweroff" utility is temporary (more of an experiment) and
will revert back to each init providing its own poweroff/reboot
commands as before.

More work needs to be done to get s6-svscan as PID 1 working 100%
(see the ugliness in "poweroff" for how this currently works).

More groundwork for new changes.