Package installer says it will reboot, but does GUI restart instead

[prl]

In Plugins.SystemPlugins.SoftwareManager.plugin the three instances where TryQuitMainLoop is run, it is run with retval=3, which does a GUI restart, but the popup that asks the user whether they want to restart says that it will reboot the system.

The message should be consistent with the action.

[Peter Urbanec]

Use correct return code to cause a reboot, rather than just GUI restart

Fixes #458

→ <<cset e48f2de009b1>>