Possible soundness issue in JobPort::Drop

[gigaroby]

While conducting an internal unsafe review, we identified a possible soundness issue in JobPort::Drop

https://github.com/watchexec/command-group/blob/b88296fed10b18aebb7ad5889dab5e43b621297f/src/winres.rs#L35

This is highly suspect when combined with #[derive(Clone)] on the struct: any use of Clone is likely to lead to a double-free. This will usually produce a harmless "invalid handle" error which this code will silently ignore, but if either HANDLE value is reused by the kernel before the second clone is dropped, undefined behavior seems possible.

[passcod]

Huh, yeah, that does look likely. Unfortunately very busy elsewhere currently but if you want to submit a PR to clear that I'll merge and release asap.