doc: link to man pages

This changes the doc generator to automatically link references such as
`open(2)` to a man page on man7.org or freebsd.org

PR-URL: https://github.com/nodejs/node/pull/5073


Reviewed-By: Ben Noorhduis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>