nspawn: be less liberal about creating bind mount destinations

nspawn: be less liberal about creating bind mount destinations

Previously, if a file's bind mount destination didn't exist, nspawn
would blindly create a directory, and the subsequent bind mount would
fail. Examine the filetype of the source and ensure that, if the
destination does not exist, that it is created appropriately.

Also go one step further and ensure that the filetypes of the source
and destination match.