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/server_test.go

@@ -519,6 +519,68 @@ func TestDuplicateNick(t *testing.T) {
 	assertContains(t, lines, " 433 ", "ERR_NICKNAMEINUSE")
 }
 
+// TestNickRefused checks that NICK, before and after
+// registration, answers a nick the HTTP API refuses with
+// ERR_ERRONEUSNICKNAME and keeps the nick as it was, and
+// that another member of the channel sees no line from it.
+func TestNickRefused(t *testing.T) {
+	t.Parallel()
+
+	// Each is what follows "NICK " on the wire.
+	refused := []string{
+		":bob PRIVMSG #inj :hi", // contains spaces
+		":",                     // empty
+		strings.Repeat("a", 33), // one character too long
+		"bob!x",
+		"bob@x",
+		"#bob",
+	}
+
+	env := newTestEnv(t)
+
+	// Before registration: the 432 is addressed to the nick
+	// sent first, and USER registers with that nick.
+	early := env.dial(t)
+	early.send("NICK early")
+
+	for _, param := range refused {
+		lines := early.sendAndExpect("NICK "+param, " 432 ")
+		assertContains(t, lines, " 432 early ", "before: "+param)
+	}
+
+	welcome := early.sendAndExpect("USER early 0 * :Test", " 001 ")
+	assertContains(t, welcome, " 001 early ", "registered as early")
+
+	// After registration.
+	bob := env.dial(t)
+	bob.register("bob")
+	bob.joinAndDrain("#inj")
+
+	mallory := env.dial(t)
+	mallory.register("mallory")
+	mallory.joinAndDrain("#inj")
+
+	bob.readUntil(func(l string) bool {
+		return strings.Contains(l, "JOIN") &&
+			strings.Contains(l, "mallory")
+	})
+
+	for _, param := range refused {
+		lines := mallory.sendAndExpect("NICK "+param, " 432 ")
+		assertContains(t, lines, " 432 mallory ", "after: "+param)
+	}
+
+	mallory.send("PRIVMSG #inj :still mallory")
+
+	bobLines := bob.readUntil(func(l string) bool {
+		return strings.Contains(l, "still mallory")
+	})
+	assertNotContains(t, bobLines, " NICK ", "bob sees no NICK")
+	assertContains(
+		t, bobLines, ":mallory!", "bob sees mallory's message",
+	)
+}
+
 func TestListChannels(t *testing.T) {
 	t.Parallel()