Require an explicit `*` network suffix for multi-upstream

Most users will connect to their server with `<username>` as their
username in order to configure their upstreams.

Multi-upstream can be unintuitive to them and should not be enabled on
that first connection that is usually used for upstream configuration.

Multi-upstream is instead a power-user feature that should be explicitly
enabled with a specific network suffix.

We reserve the network suffix `*` and use it a special case to mean that
it requests multi-upstream mode.

doc/getting-started.md

@@ -36,7 +36,7 @@ In this mode, a single connection to your soju bouncer can handle multiple
 upstream connections. You will need to manually configure each upstream
 connection using the the special `BouncerServ` user.
 
-Connect to your soju server by specifying your soju username in the username
+Connect to your soju server by specifying `<soju username>/*` in the username
 field in your client and your password in the password field.
 
 You should now be able to send private messages to the `BouncerServ`. You can

doc/soju.1.scd

@@ -39,7 +39,8 @@ soju supports two connection modes:
 - Multiple upstream mode: one downstream connection maps to multiple upstream
   connections. Channels and nicks are suffixed with the network name. To join
   a channel, you need to use the suffix too: _/join #channel/network_. Same
-  applies to messages sent to users.
+  applies to messages sent to users. To enable this mode, connect to the bouncer
+  with the username "<username>/*".
 
 For per-client history to work, clients need to indicate their name. This can
 be done by adding a "@<client>" suffix to the username.

downstream.go

@@ -1368,6 +1368,17 @@ func (dc *downstreamConn) welcome(ctx context.Context) error {
 	remoteAddr := dc.conn.RemoteAddr().String()
 	dc.logger = &prefixLogger{dc.srv.Logger, fmt.Sprintf("user %q: downstream %q: ", dc.user.Username, remoteAddr)}
 
+	if dc.networkName == "*" {
+		if !dc.srv.Config().MultiUpstream {
+			return ircError{&irc.Message{
+				Command: irc.ERR_PASSWDMISMATCH,
+				Params:  []string{dc.nick, fmt.Sprintf("Multi-upstream mode is disabled on this server")},
+			}}
+		}
+		dc.networkName = ""
+		dc.isMultiUpstream = true
+	}
+
 	// TODO: doing this might take some time. We should do it in dc.register
 	// instead, but we'll potentially be adding a new network and this must be
 	// done in the user goroutine.
@@ -1375,10 +1386,6 @@ func (dc *downstreamConn) welcome(ctx context.Context) error {
 		return err
 	}
 
-	if dc.network == nil && !dc.caps.IsEnabled("soju.im/bouncer-networks") && dc.srv.Config().MultiUpstream {
-		dc.isMultiUpstream = true
-	}
-
 	dc.updateSupportedCaps()
 
 	isupport := []string{

service.go

@@ -460,6 +460,9 @@ func (fs *networkFlagSet) update(network *Network) error {
 		network.Addr = *fs.Addr
 	}
 	if fs.Name != nil {
+		if *fs.Name == "*" {
+			return fmt.Errorf("the network name %q is reserved for multi-upstream mode", *fs.Name)
+		}
 		network.Name = *fs.Name
 	}
 	if fs.Nick != nil {