buffer: add .from(), .alloc() and .allocUnsafe()

Several changes:

* Soft-Deprecate Buffer() constructors
* Add `Buffer.from()`, `Buffer.alloc()`, and `Buffer.allocUnsafe()`
* Add `--zero-fill-buffers` command line option
* Add byteOffset and length to `new Buffer(arrayBuffer)` constructor
* buffer.fill('') previously had no effect, now zero-fills
* Update the docs

PR-URL: https://github.com/nodejs/node/pull/4682


Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>