feat: unix sockets

[YoDaMa]

Related to: #1040
This PR is still in the works, but carrys over from @ralight's PR for Unix Domain Sockets support for MQTT.js. I see there is lots of discussion around MQTT.js in Mosquitto that @ralight has participated in, and the resolution was something along the lines of, there is an actionable and useful security gain for having unix domain socket support for a service like mosquitto, so they are looking to add it.

From the MQTT.js perspective, this should be a minimal change. This PR is not complete quite yet, I think I have added some of the changes to make it work, but I want to add a unix domain sockets sample and also add the tests to support it.

For the tests, it should be very simple, just adding on a new flavor of the existing MQTT-Connection based server, except in this case listening on a unix domain port. There should be a check that the tests are running on a POSIX OS (i.e. not Windows) since obviously the tests will fail without the support of unix domain sockets on the OS.

[jdiamond]

Is file: in the URL flexible enough? It might be rare, but what if you want to use web sockets over a unix domain socket instead of plain MQTT? Or mqtts or wss?

Check out how the request library uses unix domain sockets:

https://www.npmjs.com/package/request#unix-domain-sockets

Not sure if that format is standardized and other tools use it or not.

curl supports unix sockets, too, but you use a normal URL and a separate --unix-socket option to specify the path to the socket file.

[OmgImAlexis]

@jdiamond is right, you can use http/ws with a unix socket just the same as you can a port. So really this should just use http/ws as the protocol and then a socket or port connection should depend on the connection url as the request library does.

[ralight]

The format that the request library use is also used by nginx, so it's not exactly an uncommon format.

[natcl]

Why has this been closed ?
support for unix sockets would be great !