manager: rework manager_clean_environment()

manager: rework manager_clean_environment()

Let's rename it manager_sanitize_environment() which is a more precise
name. Moreover, sort the environment implicitly inside it, as all our
callers do that anyway afterwards and we can save some code this way.

Also, update the list of env vars to drop, i.e. the env vars we manage
ourselves and don't want user code to interfear with. Also sort this
list to make it easier to update later on.