Pass -x and -v set options from main.sh to prepare.sh (useful for debugging).

I find this useful for debugging (instead of modifying `prepare.sh`'s first line manually, and reverting before a commit): you just call `sh -x -e installer/main.sh ...` and get traces not only in main.sh, but also in prepare.sh. It could be useful for bug reports as well.

It was a bit easier to implement before you added the `-xx` option to `enter-chroot` (it's a bit more "invasive" now).