Stop removing environment variables to fix problems and instead nuke
them all from orbit, it's the only way to be sure.

Less flippantly, this adds a helper function to the env utility to
restore an environment variable to its current state after it's been
cleared.  Then build_environment clears all environment variables, and
restores only those variables that we explicitly allow to persist.

This solves much of the fragility of building packages that take poorly
named common environment variables silently as arguments, such as
OpenSSL, but will likely cause breakage for cases where packages
happened to accidentally work due to re-using external state that will
no longer be visible.

…ly to env vars that are otheriwse cleared

…ystems without pwd module

…t synced)