Inplace import improvements (rebased)

Fix for https://trello.com/c/6Wv0H2Rk/126-better-explain-inplace-user-requirements - explaining extra user and group set-up needed for in-place imports.

--rebased-from #1022 (without the commit to fix typos which don't exist on this branch)
Do we simply want `-R` for the `chmod` and `chgrp`, or do insufficient Unices offer that option?