Adds support for UNIX sockets

[anarthal]

close #246

[mzimbres]

Many thanks. I will try to finish the review today after kids go to bed :) I think we have to agree on naming, I am more used to the term Unix Domain socket (UDS) than UNIX sockets. I don't know which is better or more popular, I will look at how other libraries name it.

[anarthal]

Many thanks. I will try to finish the review today after kids go to bed :) I think we have to agree on naming, I am more used to the term Unix Domain socket (UDS) than UNIX sockets. I don't know which is better or more popular, I will look at how other libraries name it.

I think the correct term is UNIX domain socket. However, I didn't immediately recognize the UDS acronym as 'UNIX domain socket', so I'd use the full term but avoid 'UDS'.

As a reference:

• The Redis server uses unixsocket and unixsocketperm as the config key names to set up UNIX domain sockets.
• UNIX itself uses AF_UNIX for the address family.
• redis-py uses unix_socket_path (Connect via Unix Socket and Connection Pool redis/redis-py#624)
• The Rust library does not have a parameter per-se, but uses the "UNIX socket" term: https://docs.rs/redis/latest/redis/#connection-parameters

I can rename config::unix_socket to config::unix_domain_socket or to config::unix_socket_path, and will update all comments to use "UNIX domain sockets"