Add a possibility to explicitly override the soname during initialization

[SanderVocke]

For purposes of mock testing, it would be very useful if I could explicitly override the .so name when initializing the dynamic loader.

This PR keeps all the existing behavior and initialize_{library} still works as before. But it also introduces a initialize_{library}_explicit where the caller can explicitly pass the .so name.

Please consider merging this as I think it adds some useful functionality while having low impact/risk.

[hpvb]

This generally looks good, but the other features of the generator are document with the examples, this one is not. Could you add a trivial example showing how this is supposed to be used?

If that is in place I'd be happy to merge this!