fix(msi): force termination of stuck processes

fix(msi): force termination of stuck processes

Stopping the Windows Explorer process does not always work.  Per default
the MSI Restart Manager will display a warning message after a timeout,
and not attempt to restart Explorer afterwards -- resulting in a black
screen if no other window is open.

Setting the property `MSIRMSHUTDOWN` to `1` will force the termination
of the process when the timeout is reached, which finally allows the
Explorer to be restarted.

Signed-off-by: Jyrki Gadinger <nilsding@nilsding.org>