bpo-47151: Fallback to fork when vfork fails in subprocess. (GH-32186)

bpo-47151: Fallback to fork when vfork fails in subprocess. An OS kernel can specifically decide to disallow vfork() in a process. No need for that to prevent us from launching subprocesses.
(cherry picked from commit 4a08c4c469d36f99d3a5e0f17ad82ab35dcf2835)

Co-authored-by: Gregory P. Smith <greg@krypto.org>