Add bus-driverd

systemd-bus-driverd is a small daemon that connects to kdbus and
implements the org.freedesktop.DBus interface. IOW, it provides the bus
functions  traditionally taken care for by dbus-daemon.

Calls are proxied to kdbus, either via libsystemd-bus (were applicable)
or with the open-coded use of ioctl().

Note that the implementation is not yet finished as the functions to
add and remove matches and to start services by name are still missing.