Allow both TCP/IP and Unix Domain socket connections

[dvader]

I would like other programs on the same machine that runs mbusd to be able to access the Modbus backend. I could use the Ethernet loopback but it seems heavy. A better way would be to allow an inter process communication to happen. Unix Domain socket seem like the most logical IPC as it is very similar to the TCP sockets without the networking overhead. It also uses a lot of the same functions so the unique code between the two socket type could be minimal.

[dgoo2308]

@dvader @3cky out of curiosity I looked into it and I have an implementation ready that should work, but .....
do you have any modbus-clients that can connect to a domain socket?