http: llhttp opt-in insecure HTTP header parsing

Allow insecure HTTP header parsing. Make clear it is insecure.

See:
- https://github.com/nodejs/node/pull/30553
- https://github.com/nodejs/node/issues/27711#issuecomment-556265881
- https://github.com/nodejs/node/issues/30515

PR-URL: https://github.com/nodejs/node/pull/30567
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/_http_common.js

@@ -28,6 +28,8 @@ const {
 const { setImmediate } = require('timers');
 
 const { methods, HTTPParser } = internalBinding('http_parser');
+const { getOptionValue } = require('internal/options');
+const insecureHTTPParser = getOptionValue('--insecure-http-parser');
 
 const FreeList = require('internal/freelist');
 const incoming = require('_http_incoming');
@@ -237,6 +239,16 @@ function prepareError(err, parser, rawPacket) {
     err.message = `Parse Error: ${err.reason}`;
 }
 
+let warnedLenient = false;
+
+function isLenient() {
+  if (insecureHTTPParser && !warnedLenient) {
+    warnedLenient = true;
+    process.emitWarning('Using insecure HTTP parsing');
+  }
+  return insecureHTTPParser;
+}
+
 module.exports = {
   _checkInvalidHeaderChar: checkInvalidHeaderChar,
   _checkIsHttpToken: checkIsHttpToken,
@@ -249,5 +261,6 @@ module.exports = {
   parsers,
   kIncomingMessage,
   HTTPParser,
+  isLenient,
   prepareError,
 };