Add support for IRC address schemes

This is preparatory work for adding other connection types to upstream
servers. The service command `network create` now accepts a scheme in
the address flag, which specifies how to connect to the upstream server.

The only supported scheme for now is ircs, which is also the default if
no scheme is specified. ircs connects to a network over a TLS TCP
connection.

doc/soju.1.scd

@@ -93,7 +93,12 @@ abbreviated form, for instance *network* can be abbreviated as *net* or just
 	the command.
 
 *network create* *-addr* <addr> [options...]
-	Connect to a new network at _addr_. _-addr_ is mandatory. Other options are:
+	Connect to a new network at _addr_. _-addr_ is mandatory.
+
+	_addr_ supports several connection types:
+	- _[ircs://]host[:port]_ connects with TLS over TCP
+
+	Other options are:
 
 	*-name* <name>
 		Short network name. This will be used instead of _addr_ to refer to the

service.go

@@ -203,6 +203,15 @@ func handleServiceCreateNetwork(dc *downstreamConn, params []string) error {
 		return fmt.Errorf("flag -addr is required")
 	}
 
+	if addrParts := strings.SplitN(*addr, "://", 2); len(addrParts) == 2 {
+		scheme := addrParts[0]
+		switch scheme {
+		case "ircs":
+		default:
+			return fmt.Errorf("unknown scheme %q (supported schemes: ircs)", scheme)
+		}
+	}
+
 	for _, command := range connectCommands {
 		_, err := irc.ParseMessage(command)
 		if err != nil {

upstream.go

@@ -66,15 +66,33 @@ type upstreamConn struct {
 func connectToUpstream(network *network) (*upstreamConn, error) {
 	logger := &prefixLogger{network.user.srv.Logger, fmt.Sprintf("upstream %q: ", network.Addr)}
 
-	addr := network.Addr
-	if !strings.ContainsRune(addr, ':') {
-		addr = addr + ":6697"
+	var scheme string
+	var addr string
+
+	addrParts := strings.SplitN(network.Addr, "://", 2)
+	if len(addrParts) == 2 {
+		scheme = addrParts[0]
+		addr = addrParts[1]
+	} else {
+		scheme = "ircs"
+		addr = addrParts[0]
 	}
 
 	dialer := net.Dialer{Timeout: connectTimeout}
 
-	logger.Printf("connecting to TLS server at address %q", addr)
-	netConn, err := tls.DialWithDialer(&dialer, "tcp", addr, nil)
+	var netConn net.Conn
+	var err error
+	switch scheme {
+	case "ircs":
+		if !strings.ContainsRune(addr, ':') {
+			addr = addr + ":6697"
+		}
+
+		logger.Printf("connecting to TLS server at address %q", addr)
+		netConn, err = tls.DialWithDialer(&dialer, "tcp", addr, nil)
+	default:
+		return nil, fmt.Errorf("failed to dial %q: unknown scheme: %v", addr, scheme)
+	}
 	if err != nil {
 		return nil, fmt.Errorf("failed to dial %q: %v", addr, err)
 	}