workaround for issues with opening data channels after data channels have closed

Bah, a nuisance to get this working right. I think the situation can be summed as follows:

> If an `RTCPeerConnection` has ever opened a data channel and any data channel has been closed (by either peer) then the *next* call to `createDataChannel` will silently fail.

So, we need to inject a dummy call to `createDataChannel`.

Tested with the SOCKS server in uproxy-networking.
Doesn't this leave a lot of data channels open in the background (all with the same label...). Do you know any more about the issue with closing the fake channel? I thought it was just problematic like a normal close, so you could close the dummy channel before closing any real channel. Then right after you can open it again...?