Cleanup:  - POSIX_SPAWN_CLOEXEC_DEFAULT is a constant offered by the Darwin module. Import it conditionally that way.  - Do not close Pipe/FileHandle-owned fds.
Cleanup:
 - POSIX_SPAWN_CLOEXEC_DEFAULT is a constant offered by the Darwin module. Import it conditionally that way.
 - Do not close Pipe/FileHandle-owned fds.
apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation