CxxUtils: Fix coverity warnings.

Scott Snyderrequested to merge
ssnyder/athena:cov.CxxUtils-20250627 into main
  • Use move instead of copy (CID 11722, 12215, 14969, 13035)
  • Check for error return from sysconf. (CID 12405, 11081)
  • Check return values (CID 10122, 10405)
  • Uninitialized struct field (CID 10095, 10351)
  • Using lock objects as temporaries seems to confuse coverity, making it miss seeing the release of the mutex. Change to avoid using temporaries. (CID 12513, 12854, 13396, 14033)
  • Avoid allocating large objects on the stack (CID 10209)
  • Missing lock in test (CID 12617, 11127)

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