expose current Eio backend in use as a string variable

[avsm]

It's currently not possible to log which Eio backend is in use, and we should expose a way to do this (potentially from Eio_main, but elsewhere is fine too).

Reported by @quernd

[talex5]

Yes, something like backend_id : string in Stdenv.base should do it.

[bord-o]

Hey this would be my first contribution, but I can take this on if that's ok. I already have a working version.

[talex5]

@bord-o: sounds good - feel free to open a PR!

[talex5]

Done in #560.