usr_sbin/grml-chroot: Fix a couple of shellcheck warnings

- SC2086: Double quote to prevent globbing and word splitting.
- SC2145: Argument mixes string and array. Use * or separate argument.
- SC2086: Double quote to prevent globbing and word splitting.
- SC2155: Declare and assign separately to avoid masking return values.
- SC2086: Double quote to prevent globbing and word splitting.
- SC2004: $/${} is unnecessary on arithmetic variables.
- SC2230: which is non-standard. Use builtin 'command -v' instead.
- SC2236: Use -n instead of ! -z.