Fix leaking orphan processes on macOS/Linux

[zafnz]

Process.start can leak orphaned child processes that sleep forever. When ProcessStarter::Start() encounters an error after fork() but before signalling the child to proceed, the cleanup path (CleanupAndReturnError) closes pipes but never kills the forked child. The child blocks forever in read(), waiting for a "go" signal that will never arrive.

This is exacerbated by a secondary issue: the child inherits the write end of its own signal pipe, so even when the parent closes its copy, the pipe doesn't break, the child's read() never returns EOF.

See #62781 for a full breakdown of everything.

Closes: #62781

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