Add a clean_environment call to libutil.

Add a clean_environment call to libutil.

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

Idea from: Jacques Vidrine

MFC after: 2 weeks