Do not unnecessarily strdup() environment variables

This reverts the memory duplication introduced in commit 60a2627, while
keeping everything else that has been cleaned up. The environment
variables are never modified, so we do not need to call xstrdupn() here.

Also, remove xstrdupn() which is no longer needed.

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>