Add set_local_credentials support

[superdump]

Would it be possible to cherry pick over this commit from upstream: http://cgit.freedesktop.org/libnice/libnice/commit/?id=4e4af4be3890f12e300fe71104564171091b7a17 ?

[superdump]

I have quickly rebased the patch here: https://github.com/superdump/libnice/tree/pexip

Building and manually testing a bit now. Not really ready for a merge though.