Over in https://bugs.python.org/issue33053#msg314192, I came up with `--basepath <dir>` as another possible spelling (`--no-basepath` would then be an option for turning it off.

The main argument *against* that name is that we use "base" to mean something different in the `sys` module: "base_prefix" and "base_exec_prefix" refer to the info for the parent environment when in a virtual environment.

Another alternative would be "--mainpath", since what we're really overriding is the default path entry that's normally derived from the location of `__main__`. As an added bonus, we could define "main path" as a glossary entry, rather than having to talk about "sys dot path zero" all the time.