fs: support pseudofiles in promises.readFile

PR-URL: https://github.com/nodejs/node/pull/21497
Fixes: https://github.com/nodejs/node/issues/21331
Refs: http://pubs.opengroup.org/onlinepubs/9699919799/functions/read.html
Refs: https://groups.google.com/forum/#!topic/nodejs-dev/rxZ_RoH1Gn0


Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>