fork+exec fails to close other file descriptors

[iCharlesHu]

(manually cloned from iCharlesHu/Subprocess#44)

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.

Implementation here: https://github.com/swiftlang/swift-sdk-generator/pull/156/files#diff-456340af052c33f5e1551237a5b2b7a6f20c374fea29d0d352c931b328a319a4R134-R140