worker: file creation races with process.umask()

This code is unsafe when worker threads are active:

	old = umask(0);
	umask(static_cast<mode_t>(old));

The umask(0) call temporarily changes the process-wide umask and races with fs operations from other threads.

Test case:

'use strict';
const { Worker, isMainThread } = require('worker_threads');
const { statSync, writeFileSync, unlinkSync } = require('fs');

function pummel() {
  for (let i = 0; i < 1e4; i++) process.umask();
  setImmediate(pummel);
}

if (isMainThread) {
  process.umask(0o22);
  new Worker(__filename);
  pummel();
} else {
  const file = 'x.txt';
  for (;;) {
    writeFileSync(file, 'ok', { mode: 0o666 });
    const s = statSync(file);
    s.mode &= 0o777;
    if (0o644 !== s.mode) throw 'unexpected mode: ' + s.mode.toString(8);
    unlinkSync(file);
  }
}

Fails within a few iterations with unexpected mode: 666

process.umask() (no arg) is allowed in workers so this test case works both ways.

This bug is potentially a security issue.