No GUI after Kubuntu 18.04 LTS upgrade from 17.10

TL;DR; Run sudo apt-get install lightdm to let the system discover and install missing dependencies. In this case, the upgrade appears to have removed bindfs, so it had to be re-installed.

It turns out that the lightdm package was left in a broken state after upgrading. The sudo dpkg-reconfigure lightdm command ended up exiting with an error and saying that the lightdm package was broken or not completely installed.

So, running sudo apt-get install lightdm again after the upgrade had completed showed that bindfs was not installed. While, according to apt-cache show lightdm, the bindfs package shows up under the 'Suggests' category, letting the process install the package actually allowed the service to run as expected and fixed the problem.

It seems that this package is actually required by lightdm to work, but is (incorrectly) not listed as such, so lightdm ends up in a broken state after upgrading.

To fix this, just re-install it to make sure dependencies are met.