[3.13] gh-126631: gh-137996: fix pre-loading of `__main__` (GH-135295) (#138609)

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.

---------
(cherry picked from commit 0912b3a6dbd226bea79eb8d70d5a06230804d4cb)

Co-authored-by: Duane Griffin <duaneg@dghda.com>