Bug#910377: Inhibit reboot/shutdown if dpkg is running

[Ansgar]

Laurent Bigonville writes:
> I'm reopening this issue and reassigning it dpkg package, that would
> at least avoid non privileged users to restart the machine when there
> is an update happening.

For what it's worth apt supports this already when apt invokes dpkg:

```C++
         if (_config->FindB("DPkg::Inhibit-Shutdown", true))
            Fd = Inhibit("shutdown", "APT", "APT is installing or removing packages", "block");
```

Ansgar