Interactive / ScalaDoc housekeeping

This is a re-issue of https://github.com/scala/scala/pull/3903 (closed during my leave)

Those are the leftovers of what should have been a larger bugfix PR, but that's finally not meant to be.
No issue, since the changes are straightforward & compiler warnings-driven, but I'd be happy to create one.
Should probably be merged into 2.12.

[reviewed & LGTM'ed](https://github.com/scala/scala/pull/3903#issuecomment-50616549) by @dragos 
Adaptation PR for refactoring is [open](https://github.com/scala-ide/scala-refactoring/pull/60)

Edit : non-trivial changes in the last commit post-review (but going in the direction of simplicity)