This function removes all environment variables except
the ones listed on a "whitelist."
The function accepts two whitelist arguments.
If the first is NULL, a built-in default list will be
used. This allows callers to get a variety of behaviors:
* Default screening: provide NULL for both lists
* Custom screening: provide a custom list for the first argument
* Modified default screening: provide NULL for first arg,
list of additional variables to preserve in the second arg