ipn/localapi: stop logging "broken pipe" errors (#18487)

The Tailscale CLI has some methods to watch the IPN bus for
messages, say, the current netmap (`tailscale debug netmap`).
The Tailscale daemon supports this using a streaming HTTP
response. Sometimes, the client can close its connection
abruptly -- due to an interruption, or in the case of `debug netmap`,
intentionally after consuming one message.

If the server daemon is writing a response as the client closes
its end of the socket, the daemon typically encounters a "broken pipe"
error. The "Watch IPN Bus" handler currently logs such errors after
they're propagated by a JSON encoding/writer helper.

Since the Tailscale CLI nominally closes its socket with the daemon
in this slightly ungraceful way (viz. `debug netmap`), stop logging
these broken pipe errors as far as possible. This will help avoid
confounding users when they scan backend logs.

Updates #18477

Signed-off-by: Amal Bansode <amal@tailscale.com>

net/neterror/neterror_js.go

@@ -0,0 +1,20 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build js || wasip1 || wasm
+
+package neterror
+
+import (
+	"errors"
+	"io"
+	"io/fs"
+)
+
+// Reports whether err resulted from reading or writing to a closed or broken pipe.
+func IsClosedPipeError(err error) bool {
+	// Libraries may also return root errors like fs.ErrClosed/io.ErrClosedPipe
+	// due to a closed socket.
+	return errors.Is(err, fs.ErrClosed) ||
+		errors.Is(err, io.ErrClosedPipe)
+}