installer: Add support for the /portable parameter

Actually support /portable which is already being used by install-here.sh.

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
kill-shell: Kill child processes manually

This avoids taskkill returning an error when trying to kill a child
process that was implicitly killed before.

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>