src: introduce internal Buffer::Copy() function

Rename the three argument overload of Buffer::New() to Buffer::Copy()
and update the code base accordingly.  The reason for renaming is to
make it impossible to miss a call site.

This coincidentally plugs a small memory leak in crypto.getAuthTag().

Fixes: https://github.com/nodejs/node/issues/2308
PR-URL: https://github.com/nodejs/node/pull/2352


Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>