Remove some compilation warnings

In practice, these functions' results are rarely used, and the
"ignoring return value" warnings they cause are just noise.

 * 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.

 * They may return job IDs, for future reference.  In practice,
   callers don't bother recording them.