buffer: backport --zero-fill-buffers command line option

This backports the --zero-fill-buffers command line flag introduced
in master. When used, all Buffer and SlowBuffer instances will zero
fill by default.

This does *not* backport any of the other Buffer API or behavior
changes.

PR-URL: https://github.com/nodejs/node/pull/5744


Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>