child_process: measure buffer length in bytes

This change fixes a known issue where `maxBuffer` limits by characters
rather than bytes. Benchmark added to confirm no performance regression
occurs with this change.

PR-URL: https://github.com/nodejs/node/pull/6764
Fixes: https://github.com/nodejs/node/issues/1901


Reviewed-By: Brian White <mscdex@mscdex.net>