Adjustments to debian template for systemd

Among the changes are:

* mask systemd-udevd.service in addition to udev.service (jessie name and wheezy name, respectively)
* try to configure as much of the systemd stuff as possible even when systemd is not installed, in anticipation for upgrades and init system switches
* use `update-rc.d disable` instead of `remove`, as the effects of remove are reset after the packages are updated and `defaults` is run

I do have a question: why is there no /dev/tty0 device when `lxc.autodev` is enabled? If there was, we could get rid of at least some of the ugly sed hack.