unix domain sockets only work in headless mode without --continue flag

[Fabianexe]

In #3655 the option that unix domain sockets are can be used as listen would be added.
By testing this I have notice that it only work in headless mode without --continue flag.

The reason is that listener.Addr().String() is used as input to rpc2.NewClient and connect in the execute command. However, the String method on a unix address (net.UnixAddr) only returns the path without the network ("unix:") as prefix.

Further the rpc2.NewClient does connection again directly with a "tcp" network without checking for the prefix before.