* Added support for shells without default variable support
* Added support for BusyBox installs that do not have `command` but has `which`
* Style fixes as suggested by reviewer
* Renamed `WHERE_IS` to `EXISTS` as suggested by review
* Removed expansion of `$LE_AUTO_SUDO` to `x` as the `-n` can check empty strings.
* Added `EXISTS` to debian bootstrap as suggested in review
