net/netutil: fix regression where peerapi would get closed after 1st req

I introduced a bug in 8fe503057d when unifying oneConnListener
implementations.

The NewOneConnListenerFrom API was easy to misuse (its Close method
closes the underlying Listener), and we did (via http.Serve, which
closes the listener after use, which meant we were close the peerapi's
listener, even though we only wanted its Addr)

Instead, combine those two constructors into one and pass in the Addr
explicitly, without delegating through to any Listener.

Change-Id: I061d7e5f842e0cada416e7b2dd62100d4f987125
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/netutil/netutil_test.go

@@ -0,0 +1,54 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package netutil
+
+import (
+	"io"
+	"net"
+	"testing"
+)
+
+type conn struct {
+	net.Conn
+}
+
+func TestOneConnListener(t *testing.T) {
+	c1 := new(conn)
+	a1 := dummyAddr("a1")
+
+	// Two Accepts
+	ln := NewOneConnListener(c1, a1)
+	if got := ln.Addr(); got != a1 {
+		t.Errorf("Addr = %#v; want %#v", got, a1)
+	}
+	c, err := ln.Accept()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if c != c1 {
+		t.Fatalf("didn't get c1; got %p", c)
+	}
+	c, err = ln.Accept()
+	if err != io.EOF {
+		t.Errorf("got %v; want EOF", err)
+	}
+	if c != nil {
+		t.Errorf("unexpected non-nil Conn")
+	}
+
+	// Close before Accept
+	ln = NewOneConnListener(c1, a1)
+	ln.Close()
+	_, err = ln.Accept()
+	if err != io.EOF {
+		t.Fatalf("got %v; want EOF", err)
+	}
+
+	// Implicit addr
+	ln = NewOneConnListener(c1, nil)
+	if ln.Addr() == nil {
+		t.Errorf("nil Addr")
+	}
+}