List the user modes in the 004 reply (closes #220)

Both transports sent an empty string as the user modes in RPL_MYINFO.
On the IRC listener that left an empty parameter in the middle of the
line, so a client that splits on spaces read the channel modes as the
user modes. The reply now lists o and w, the user modes the server
supports, from one constant in pkg/irc that both transports read. The
README and the schema notes now show what the HTTP API sends.

Model: opus-5-5

pkg/irc/numerics.go

@@ -65,6 +65,11 @@ const (
 	RplIsupport IRCMessageType = 5 // De-facto standard (same numeric as RplBounce)
 )
 
+// UserModes lists the user modes the server supports, as
+// RPL_MYINFO sends them: o (server operator) and w
+// (receives WALLOPS).
+const UserModes = "ow"
+
 // Command responses (200-399).
 const (
 	// RFC 2812 trace/stats/links replies (200-219).