Emulate POSIX_SPAWN_CLOEXEC_DEFAULT in fork/exec.

POSIX_SPAWN_CLOEXEC_DEFAULT is only available on Darwin. Emulate POSIX_SPAWN_CLOEXEC_DEFAULT on other platforms by calling close after fork, before exec.

This commit also removes _subprocess_posix_spawn_fallback because we can't emulate POSIX_SPAWN_CLOEXEC_DEFAULT in a thread-safe manner while using posix_spawn.

Author: iCharlesHu