Translate signal numbers to user-friendly strings

[vorner]

Signal hook uses the low-level c_int to represent signal numbers, because signal numbers are somehow "open" enum ‒ it's not clear what signals might be available on whatever platform someone compiles signal-hook on, or if they are having a custom kernel patch for their embedded device.

But if one wants to present the signal to the user (maybe in the logs, or something), printing the number is not the most user-friendly thing one can do. It would be nice if:

• We had some way to convert that to user-friendly name (either having an enum with TryFrom<c_int> on it, or some kind of fn(c_int) -> &str function)
• Debug outputs of various things (especially the Origin) would do this conversion.

❓ If we decide to go with that enum type, should our API generally accept that in addition to raw c_int, in the form of Into<c_int>, or the like?