reset HOME, USER(NAME), TMPDIR & SHELL in DropPrivileges

reset HOME, USER(NAME), TMPDIR & SHELL in DropPrivileges

We can't cleanup the environment like e.g. sudo would do as you usually
want the environment to "leak" into these helpers, but some variables
like HOME should really not have still the value of the root user – it
could confuse the helpers (USER) and HOME isn't accessible anyhow.

Closes: 842877