relay: add UNIX socket support (closes #733)

doc/en/weechat_user.en.adoc

@@ -3194,6 +3194,27 @@ websocket = new WebSocket("ws://server.com:9000/weechat");
 The port (9000 in example) is the port defined in Relay plugin.
 The URI must always end with "/weechat" (for _irc_ and _weechat_ protocols).
 
+[[relay_unixsocket]]
+==== UNIX domain sockets
+
+Using the protocol option "unix" with the "/relay add"  command, you can
+listen using any protocol on a UNIX domain socket at a given path. For exmaple:
+
+----
+/relay add unix.weechat /tmp/weesock
+----
+
+will allow clients to connect using the WeeChat protocol to /tmp/weesock. This
+is particularly useful to allow SSH forwarding for relay clients, when other
+ports cannot be opened. Using OpenSSH:
+
+----
+$ ssh -L9000:/tmp/weesock foo_host
+----
+
+will then allow for local relay clients to connect on port 9000 to a WeeChat
+instance running on "foo_host".
+
 [[relay_commands]]
 ==== Commands