Bundle system configuration files in Debian package

[PhilippWendler]

The systemd unit that creates the necessary cgroups could be bundled in the Debian package such that cgroups setup is done automatically when the package is installed. The package could also create the appropriate group and advise the user to add itself to the group.

Similarly, we could put a file into /etc/sysctl.d that sets the sysctl flag that is necessary for user namespaces on Debian machines, though we have to be careful such that this also works on systems where the sysctl flag does not exist.

[PhilippWendler]

The first part is done in e18287d, but the sysctl flag and the swapaccount flag for the kernel command-line are still not set.