buffer: zero fill Buffer(num) by default

Zero-fill Buffer(num) and new Buffer(num) by default.

Refs: https://github.com/nodejs/CTC/issues/89

@nodejs/ctc

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

buffer