the case where one of several guests wants to rematch is a hard
problem for later.) Requires passing old-style relayIDs (connname plus
device index) when devIDs aren't available, which they may not always
be.
for Rematch): works for linux version, provided you know the relayID
of the device you're inviting. Added to common/ a stream-saving
version of java's NetLaunchInfo I'll probably want to use there too
for cross-platform compatibility (there being no jni support for
json.)
