fork+exec fails to close other file descriptors

https://github.com/iCharlesHu/swift-experimental-subprocess/blob/7204d94bc468c0669a326fe584bf240e1eecbb2b/Sources/_SubprocessCShims/process_shims.c#L322

UNIX programs typically need to close _all other open_ fds that they don't want to inherit into the child process after `fork()` but before `execve()`. This code fails to do so, this is security-relevant.