Support dirfd relative iops on Windows

We always favor calling functions like openat(), fstatat(), etc. because
Linux, XNU, FreeBSD, and OpenBSD all elected to support them, while some
systems like Android love them so much, that they stopped supporting the
old interfaces.

This change ensures that when dirfd is actually a dirfd and not AT_FDCWD
we'll do the right thing on Windows NT. We use an API that's been around
since Vista to accomplish that.

This change also adds exponential backoff to chdir() on Windows since it
seems almost as flaky on Windows 7 as the rmdir() function.

libc/calls/mkdir.c

@@ -41,10 +41,5 @@
  * @see makedirs()
  */
 int mkdir(const char *path, unsigned mode) {
-  if (!path) return efault();
-  if (!IsWindows()) {
-    return mkdirat$sysv(AT_FDCWD, path, mode);
-  } else {
-    return mkdir$nt(path, mode);
-  }
+  return mkdirat(AT_FDCWD, path, mode);
 }