[3.10] gh-91401: Conservative backport of `subprocess._USE_VFORK` (#91932)

This does not alter the `_posixsubprocess.fork_exec()` private API to
avoid issues for anyone relying on that (bad idea) or for anyone who's
`subprocess.py` and `_posixsubprocess.so` upgrades may not become
visible to existing Python 3.10 processes at the same time.

Backports the concept of cd5726fe674eaff442510eeb6c75628858be9e9f.

Provides a fail-safe way to disable vfork for #91401.

I didn't backport the documentation as I don't actually expect this to be used and `.. versionadded: 3.10.5` always looks weird in docs. It's being done more to have a fail-safe in place for people just in case.