repl: Private Buffer object in lib/* files

Fixes usage of global object 'Buffer' in lib/* files by ensuring that
each file does an explicit require('buffer').Buffer.  Previously, when
running a repl, due to usage of global 'Buffer', any redefinition of
Buffer would cause a crash eg var Buffer = {}.

Fixes: https://github.com/joyent/node/issues/8588
PR-URL: https://github.com/joyent/node/pull/8603
Reviewed-by: Trevor Norris <trev.norris@gmail.com>