Change unix:// to irc+unix://

When Unix socket support will be added for listeners, unix:// will be
ambiguous. It won't be clear whether to setup an IRC server, or some
other kind of server (e.g. identd).

unix:// is still recognized to avoid breaking existing DBs.

doc/soju.1.scd

@@ -128,7 +128,7 @@ abbreviated form, for instance *network* can be abbreviated as *net* or just
 
 	- _[ircs://]<host>[:port]_ connects with TLS over TCP
 	- _irc+insecure://<host>[:port]_ connects with plain-text TCP
-	- _unix:///<path>_ connects to a Unix socket
+	- _irc+unix:///<path>_ connects to a Unix socket
 
 	Other options are:
 

upstream.go

@@ -142,7 +142,7 @@ func connectToUpstream(network *network) (*upstreamConn, error) {
 		if err != nil {
 			return nil, fmt.Errorf("failed to dial %q: %v", addr, err)
 		}
-	case "unix":
+	case "irc+unix", "unix":
 		logger.Printf("connecting to Unix socket at path %q", u.Path)
 		netConn, err = dialer.Dial("unix", u.Path)
 		if err != nil {