Check nicks on the IRC listener with the HTTP API's rule (closes #142)

The nick rule moves from internal/handlers to internal/service as
ValidNick, and the HTTP API and both IRC listener NICK paths call it.
The IRC listener used to cut a nick to 32 bytes and accept anything
else, so a nick with spaces such as "bob PRIVMSG #inj :hi" made the
lines built from it read as another user's PRIVMSG. It now answers a
nick the rule refuses with ERR_ERRONEUSNICKNAME (432), before and
after registration, and keeps the nick as it was.

Model: opus-5-5

internal/ircserver/conn.go

@@ -27,7 +27,6 @@ const (
 	pollInterval    = 100 * time.Millisecond
 	pingInterval    = 90 * time.Second
 	pongDeadline    = 30 * time.Second
-	maxNickLen      = 32
 	minPasswordLen  = 8
 	maxHashcashBits = 40
 )
@@ -426,11 +425,16 @@ func (c *Conn) handlePreRegistration(
 			return
 		}
 
-		c.nick = msg.Params[0]
-		if len(c.nick) > maxNickLen {
-			c.nick = c.nick[:maxNickLen]
+		if !service.ValidNick(msg.Params[0]) {
+			c.sendNumeric(
+				irc.ErrErroneusNickname,
+				msg.Params[0], "Erroneous nickname",
+			)
+
+			return
 		}
 
+		c.nick = msg.Params[0]
 		c.gotNick = true
 	case irc.CmdUser:
 		if len(msg.Params) < 4 { //nolint:mnd