Support cross-testing (version 3)

[Roland McGrath]

>  # Produce properly quoted Bourne shell arguments for 'env' to carry
>  # over the current environment, less blacklisted variables.
>  exports="$(blacklist_exports)"
> -exports="${exports:+${exports}; }"

The comment talks about 'env' but this producing sh commands, not
a single 'env' command.