Option to use local sockets on localhost

It would be nice to have the option to use unix domain sockets or windows named pipes when both client and server are on localhost. This should improve security a lot on unix systems where permission to connect can be defined by local filesystem permissions.

Perhaps this should even become the safe and secure default, with users having to opt into using TCP because it comes with security problems on multi user systems.

IIRC SSH can also open tunnels to socket files, so perhaps we could keep the tunnel-based remote access functionality by default as well. We'd need to figure out an appropriate location for the socket files within the user's home directory, or some such.

See also discussion in https://github.com/c42f/RemoteREPL.jl/issues/29#issuecomment-1007161896