src: use GetCurrentProcessId() for process.pid

Commit a9c0c65f ("src: define getpid() based on OS") made src/env.cc
use `GetCurrentProcessId()` on Windows for the PID in log messages.
`GetCurrentProcessId()` is also what is used by libuv, OpenSSL and V8.

This commit makes `process.pid` use `GetCurrentProcessId()` instead of
`_getpid()` for consistency.

PR-URL: https://github.com/nodejs/node/pull/4163


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>