tty: add ref() so process.stdin.ref() etc. work

Also squashed from:
* test: move tty-wrap isrefed test to pseudo-tty/
* test: test tty-wrap handle isrefed properly
* test: improve failure messages in isrefed tests

PR-URL: https://github.com/nodejs/node/pull/7360


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: James M Snell <jasnell.gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>