* SC1091: the `config` file might not be present, which is normal.
* SC2086: Double quote to prevent globbing and word splitting.
Tested clean output using: `find -name "*.sh" -exec shellcheck -x {} \;`
* Made more specific shellcheck disables
* Fixed variable quoting (SC2086,SC2064)
* Use `$*` expansion instead of `$@` when not using arrays (SC2124)
* Use cleaner `$()` syntax instead of back quotes (SC2006)
* Improved comparator (SC2166)
* Minor improvements in coding style
Tested clean output using: `find -name "*.sh" | xargs -n1 shellcheck -x`.
* Tidy up and simplify function code
* Allow to pass extra arguments to debootstrap, e.g. `--variant minbase`
* Preserve compatibility with existing function calls
Hugo Hromic