Modernize for current Go toolchains

The library already compiled and passed tests, but tooling and
dependencies were stale. This brings everything up to current standards
without changing public behavior.

Library (go 1.23.0):
- cmux_test.go: fix 8 `go vet` failures by sending writer-goroutine
  errors to the existing errCh (now buffered) instead of t.Fatal;
  ioutil.ReadAll -> io.ReadAll
- matchers.go: io/ioutil -> io (io.Discard); gofmt -s doc comment
- go.mod: go 1.11 -> 1.23.0; golang.org/x/net -> v0.42.0 (newest
  release that still permits a 1.23 directive)

Example submodule:
- golang.org/x/net/context -> stdlib context
- grpc v1.27.0 -> v1.80, modern protobuf; helloworld now comes from
  the separate google.golang.org/grpc/examples module (requires go 1.25,
  isolated to this demo-only module)

CI:
- remove ancient .travis.yml (Go 1.6-1.8)
- add .github/workflows/ci.yml: gofmt/vet/build/test -race for the
  library (Go 1.23 + 1.24 matrix) and the example

Temporary() methods and TLS version defaults are intentionally
unchanged: net.Error still requires Temporary(), and the TLS defaults
are public-API behavior.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

cmux_test.go

@@ -22,7 +22,6 @@ import (
 	"fmt"
 	"go/build"
 	"io"
-	"io/ioutil"
 	"log"
 	"net"
 	"net/http"
@@ -194,7 +193,7 @@ func runTestHTTPClient(t *testing.T, proto string, addr net.Addr) {
 		}
 	}()
 
-	b, err := ioutil.ReadAll(r.Body)
+	b, err := io.ReadAll(r.Body)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -321,7 +320,7 @@ func TestTimeout(t *testing.T) {
 
 func TestRead(t *testing.T) {
 	defer leakCheck(t)()
-	errCh := make(chan error)
+	errCh := make(chan error, 1)
 	defer func() {
 		select {
 		case err := <-errCh:
@@ -335,10 +334,11 @@ func TestRead(t *testing.T) {
 	writer, reader := net.Pipe()
 	go func() {
 		if _, err := io.WriteString(writer, strings.Repeat(payload, mult)); err != nil {
-			t.Fatal(err)
+			errCh <- err
+			return
 		}
 		if err := writer.Close(); err != nil {
-			t.Fatal(err)
+			errCh <- err
 		}
 	}()
 
@@ -379,7 +379,7 @@ func TestRead(t *testing.T) {
 
 func TestAny(t *testing.T) {
 	defer leakCheck(t)()
-	errCh := make(chan error)
+	errCh := make(chan error, 1)
 	defer func() {
 		select {
 		case err := <-errCh:
@@ -403,7 +403,7 @@ func TestTLS(t *testing.T) {
 	generateTLSCert(t)
 	defer cleanupTLSCert(t)
 	defer leakCheck(t)()
-	errCh := make(chan error)
+	errCh := make(chan error, 1)
 	defer func() {
 		select {
 		case err := <-errCh:
@@ -428,7 +428,7 @@ func TestTLS(t *testing.T) {
 
 func TestHTTP2(t *testing.T) {
 	defer leakCheck(t)()
-	errCh := make(chan error)
+	errCh := make(chan error, 1)
 	defer func() {
 		select {
 		case err := <-errCh:
@@ -439,10 +439,11 @@ func TestHTTP2(t *testing.T) {
 	writer, reader := net.Pipe()
 	go func() {
 		if _, err := io.WriteString(writer, http2.ClientPreface); err != nil {
-			t.Fatal(err)
+			errCh <- err
+			return
 		}
 		if err := writer.Close(); err != nil {
-			t.Fatal(err)
+			errCh <- err
 		}
 	}()
 
@@ -493,7 +494,7 @@ func testHTTP2MatchHeaderField(
 	notMatchValue string,
 ) {
 	defer leakCheck(t)()
-	errCh := make(chan error)
+	errCh := make(chan error, 1)
 	defer func() {
 		select {
 		case err := <-errCh:
@@ -505,12 +506,14 @@ func testHTTP2MatchHeaderField(
 	writer, reader := net.Pipe()
 	go func() {
 		if _, err := io.WriteString(writer, http2.ClientPreface); err != nil {
-			t.Fatal(err)
+			errCh <- err
+			return
 		}
 		var buf bytes.Buffer
 		enc := hpack.NewEncoder(&buf)
 		if err := enc.WriteField(hpack.HeaderField{Name: name, Value: headerValue}); err != nil {
-			t.Fatal(err)
+			errCh <- err
+			return
 		}
 		framer := http2.NewFramer(writer, nil)
 		err := framer.WriteHeaders(http2.HeadersFrameParam{
@@ -520,10 +523,11 @@ func testHTTP2MatchHeaderField(
 			EndHeaders:    true,
 		})
 		if err != nil {
-			t.Fatal(err)
+			errCh <- err
+			return
 		}
 		if err := writer.Close(); err != nil {
-			t.Fatal(err)
+			errCh <- err
 		}
 	}()
 
@@ -558,7 +562,7 @@ func testHTTP2MatchHeaderField(
 
 func TestHTTPGoRPC(t *testing.T) {
 	defer leakCheck(t)()
-	errCh := make(chan error)
+	errCh := make(chan error, 1)
 	defer func() {
 		select {
 		case err := <-errCh:
@@ -583,7 +587,7 @@ func TestHTTPGoRPC(t *testing.T) {
 
 func TestErrorHandler(t *testing.T) {
 	defer leakCheck(t)()
-	errCh := make(chan error)
+	errCh := make(chan error, 1)
 	defer func() {
 		select {
 		case err := <-errCh:
@@ -624,7 +628,7 @@ func TestErrorHandler(t *testing.T) {
 
 func TestMultipleMatchers(t *testing.T) {
 	defer leakCheck(t)()
-	errCh := make(chan error)
+	errCh := make(chan error, 1)
 	defer func() {
 		select {
 		case err := <-errCh:
@@ -653,7 +657,7 @@ func TestMultipleMatchers(t *testing.T) {
 
 func TestListenerClose(t *testing.T) {
 	defer leakCheck(t)()
-	errCh := make(chan error)
+	errCh := make(chan error, 1)
 	defer func() {
 		select {
 		case err := <-errCh:
@@ -698,7 +702,7 @@ func TestListenerClose(t *testing.T) {
 
 func TestClose(t *testing.T) {
 	defer leakCheck(t)()
-	errCh := make(chan error)
+	errCh := make(chan error, 1)
 	defer func() {
 		select {
 		case err := <-errCh: