Allow using unix sockets for safe usage on multi-user machines

[Ten0]

On multi-user dev machines (such as is our case), it seems there's no way to prevent one user from sending commands to another user's instance, as it's using a standard TCP port on the machine. Also, it means the users must agree on which port each of them uses, which might be tedious.

It seems that a solution to both issues would be to allow using unix sockets, which are basically sockets but path-based, supporting file permissions. It seems that using a local file in the user's home directory would naturally propagate that directory's permissions, and avoid conflicts.

[pr2502]

The reason I've originally written it with TCP sockets is that UNIX sockets don't work on windows and I wanted to make it portable.

If someone wants unix sockets and submits a PR that will allow a unix socket in the listen and connect config options without breaking windows and users who are using tcp sockets and/or are using default config I'd be happy to accept that. We could even make unix sockets the default config on *nix systems.