feat(profiles): Add systemd profile from my Gentoo systemd-only overlay.

No need for me to maintain a similar profile in two entirely different
ways. This is also one tiny step towards cleaning up our profiles in
general. Original here: https://github.com/marineam/systemd-only-overlay

As part of this change the baselayout dependency on openrc is now
handled via a use flag instead of package.provided. We didn't previously
include a virtual/init package but Gentoo has one and I needed it for my
generic systemd-only overlay so might as well include it here if it is
needed in the future.
fix(profiles): Use systemd profile, clean init stuff from other places.

Now the default/linux profile pulls in the systemd-only profile, all
other things systemd/openrc/init references can go away now.
bump(sys-kernel/dracut): bump to 031 from gentoo

Only local modification is to remove the sysvinit dependency
drop(sys-apps/sysvinit-tools): was only required by dracut < 031

031 includes a shell reimplementation of pidof which is the only tool I
could find that it used from sysvinit.
drop(sys-apps/dbus): Moved to portage-stable
fix(profiles): Disable systemd use flag for dbus on targets.

This flag enables 'at_console' policy support using logind. I don't
think we actually have a use for that and having it disabled hasn't
caused anything weird that I know of so far so leave it disabled.

Enabling this flag causes a circular dependency between systemd and dbus
which is resolved in catalyst bootstrapped builds like the SDK but for
target builds this is a problem.
Merge pull request #150 from marineam/systemd

Systemd/init/profile updates