process: improve `--redirect-warnings` handling

1) Acquiring the file descriptor is not observable anymore when using
   the `--redirect-warnings` flag.
2) If `fs.appendFile` fails, the warning is now redirected to the
   default output.
3) The code is smaller and simpler.

PR-URL: https://github.com/nodejs/node/pull/24965


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>