There is nothing temporary about our "tmpdir" - its sole purpose is to
contain the final product of the bootstrap process. Thus, removing it
at the end of bootstrap amounts to doing the entire process for nothing.
To remedy this, --tmpdir is renamed --target, keeping the -t short form,
and defaulting to "target" instead of "tmp" to make its purpose clearer.
The --preserve option is removed, as the target is now always preserved.
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