refactor(handler/http2): split handler into 3 focused files with 52 tests

Extract authenticate/probe-resistance to auth.go and roundTrip/forwarding
to proxy.go, following the handler/http/ pattern. Export sentinel errors
(ErrAuthFailed, ErrWrongConnType) for precise test assertions. Fix
flushWriter panic recovery to handle non-string/non-error panics. Default
HTTPS port to 443 when no port is specified in the Host header.

handler/http2/conn.go

@@ -2,6 +2,7 @@ package http2
 
 import (
 	"errors"
+	"fmt"
 	"io"
 	"net/http"
 )
@@ -13,11 +14,14 @@ type flushWriter struct {
 func (fw flushWriter) Write(p []byte) (n int, err error) {
 	defer func() {
 		if r := recover(); r != nil {
-			if s, ok := r.(string); ok {
-				err = errors.New(s)
-				return
+			switch v := r.(type) {
+			case string:
+				err = errors.New(v)
+			case error:
+				err = v
+			default:
+				err = fmt.Errorf("%v", v)
 			}
-			err = r.(error)
 		}
 	}()