Remove some compilation warnings
In practice, these functions' results are rarely used, and the "ignoring return value" warnings they cause are just noise.
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They may return -1 if their argument is invalid, but that would indicate a logical error in the call chain. The caller can't be expected to handle it, and the function has already logged it.
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They may return job IDs, for future reference. In practice, callers don't bother recording them.