It is now more similar to the down connection, using the onclose
callback for resource management.

Make the de-facto "comment" and "contact" fields official.

This avoids flicker upon renegotiation.

Make it possible to attach filters dynamically.  Work around bugs
in Firefox.

If we send two offers back to back, the second answer will arive in
stable state, which will confuse us.  Instead, queue the second offer.

No user interface yet.

This avoids special-casing file streams.

Thanks to Jeroen van Veen for the debugging.