Introduce Scanner interface as an abstraction for Clamd struct

[legosx]

For testing purposes I would like to have an interface as an abstraction from Clamd.

Pull Request is just adding the interface without changing any logic.

The name Scanner can be discussed but I think that it represents the methods inside of the interface.

[Freeaqingme]

Do we really need a single interface containing all methods? Would it not be better to split it up in multiple smaller logical chunks?

Wrt to your return argument. How does it pertain to the guideline of 'accept interfaces, return structs'?