Refactoring: startmode_for_product -> new_device_startmode

... and code cleanup

Most significant change in functionality is that default startmode is
handled completely in this function. It used to be solved partially
outside (when default startmode by product wasn't in {"auto",
"ifplugd"}).
Adapted testsuite to changes caused by refactoring.
Default startmode for new device in installation set to auto/hotplug

when product features requests ifplugd. It makes more sense e.g. due to
the fact that ifplugd requires additional package which is currently
not present in inst-sys and cannot be installed from online repositories
bcs ... we don't have network.
Updated changelog.
Do not display Add button in Overview dialog during installation.

User should be able to configure existing devices during installation.
This can be achieved via "Edit" button on automatically detected
devices. Advanced configuration should be postponed to installed system.