Sanatize zpios-sanity.sh environment

Just like zconfig.sh the zpios-sanity.sh tests should run in a
sanatized environment. This ensures they never conflict with an
installed /etc/zfs/zpool.cache file.

This commit additionally improves the -c cleanup option. It now
removes the modules stack if loaded and destroys relevant md devices.
This behavior is now identical to zconfig.sh.