Ignore *.wants in systemd.packages for now
dbus: Use upstream units
sshd: Always start a session

Partially reverts 70a4c7b1dfdb238d3729c3f71127538943a43afd. Whether to
start a session is independent of whether we're running in a
container.
dbus: Merge tools and daemon

This resolves a cyclic dependency: the daemon depends on tools (for
dbus-send) while tools depends on the daemon.  Keeping them separate
doesn't seem very useful in any case.
sshd: Add support for socket activation

By enabling ‘services.openssh.startWhenNeeded’, sshd is started
on-demand by systemd using socket activation. This is particularly
useful if you have a zillion containers and don't want to have sshd
running permanently. Note that socket activation is not noticeable
slower, contrary to what the manpage for ‘sshd -i’ says, so we might
want to make this the default one day.