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: #6534
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Robert Lindstaedt <robert.lindstaedt@gmail.com>

 Conflicts:
	doc/api/fs.md

"doc/api/fs.md" file had some conflict markers like "<<<<<<< HEAD"
that are visible at the bottom of https://nodejs.org/api/fs.html

PR-URL: #7590
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>

"doc/api/fs.md" file had some conflict markers like "<<<<<<< HEAD"
that are visible at the bottom of https://nodejs.org/api/fs.html

PR-URL: #7590
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>