Fix IRC listener relay of bodies and modes, and JOIN names (closes #127)

The IRC listener relayed only the first line of a PRIVMSG or NOTICE
body; it now sends one line per body line. A relayed MODE change now
carries its key, limit or mask, each value its own parameter, also in
the echo to the user who made it. CR, LF and NUL are removed from each
parameter and from each whole line sent to an IRC listener connection,
so no stored value can start a line of its own. The channel name check
moved into service.JoinChannel, so the IRC listener's JOIN refuses the
same names with 403.

Model: opus-5-5

internal/ircserver/conn.go

@@ -297,11 +297,17 @@ func (c *Conn) send(line string) {
 }
 
 // sendWithin writes a formatted IRC line to the connection,
-// giving up after timeout.
+// giving up after timeout. CR, LF and NUL are removed first,
+// as FormatMessage does for each parameter, so that a line
+// built without it cannot end early either.
 func (c *Conn) sendWithin(
 	timeout time.Duration,
 	line string,
 ) {
+	line = strings.ReplaceAll(line, "\r", "")
+	line = strings.ReplaceAll(line, "\n", "")
+	line = strings.ReplaceAll(line, "\x00", "")
+
 	c.writeMu.Lock()
 	defer c.writeMu.Unlock()