src: refactor require('constants')

The require('constants') module is currently undocumented and mashes
together unrelated constants. This refactors the require('constants')
in favor of distinct os.constants, fs.constants, and crypto.constants
that are specific to the modules for which they are relevant. The
next step is to document those within the specific modules.

PR-URL: https://github.com/nodejs/node/pull/6534


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Robert Lindstaedt <robert.lindstaedt@gmail.com>