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/commands.go

@@ -71,8 +71,13 @@ func (c *Conn) handleNick(
 	}
 
 	newNick := msg.Params[0]
-	if len(newNick) > maxNickLen {
-		newNick = newNick[:maxNickLen]
+	if !service.ValidNick(newNick) {
+		c.sendNumeric(
+			irc.ErrErroneusNickname,
+			newNick, "Erroneous nickname",
+		)
+
+		return
 	}
 
 	oldMask := c.hostmask()