build-depends leaving processes running can cause problems

[Ealdwulf]

kano-settings currently has a build dependency on kano-toolset, which has a dependency on dbus, which
runs a dbus-daemon which opens a file in /proc. This prevents dr from unmounting the chroot, which in turn means that although it built the package, it decides not to go on to push it to the suite.

Ideally dr should kill all child processes. I think this can be done simply by launching the chroot with unshare -u chroot..., whose children can then be killed using pkill --nslist uts --ns <PID> where <PID> is the pid of one of the processes inside the chroot.

@skarbat @tombettany @pazdera
For the time being I think we will fix kano-settings.