[3.14] gh-126631: gh-137996: fix pre-loading of `__main__` (GH-135295) (#138607)

[3.14] gh-126631: gh-137996: fix pre-loading of `__main__` (GH-135295) (#138607)

gh-126631: gh-137996: fix pre-loading of `__main__` (GH-135295)

gh-126631: gh-137996: fix pre-loading of `__main__`

The `main_path` parameter was renamed `init_main_from_name`, update the
forkserver code accordingly.  This was leading to slower startup times when people
were trying to preload the main module.

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(cherry picked from commit 0912b3a6dbd226bea79eb8d70d5a06230804d4cb)

Co-authored-by: Duane Griffin <duaneg@dghda.com>
Co-authored-by: Gregory P. Smith <greg@krypto.org>
Co-authored-by: Petr Viktorin <encukou@gmail.com>
Co-authored-by: Gregory P. Smith <68491+gpshead@users.noreply.github.com>