Vestigal support for UNIX sockets?

[RoganDawes]

It seems that there was a plan to support UNIX sockets at some point, since there is a definition in proto/controlmsg.go. Unfortunately, there is no support in the client (it reports "invalid protocol"), and if I add client support for the protocol in cmd/tunnel/config.go, I end up crashing the server binary and disconnecting all the other clients! I guess this is because there is no : in the remote_addr field, and it tries to substring from a negative index or some such.

Is there any information on how this was intended to be used? Was there ever proper support for UNIX sockets?