child_process: control argv0 for spawned processes

In some cases it useful to control the value of `argv[0]`, c.f.
 - https://github.com/andrewffff/child_process_with_argv0
 - https://github.com/andrep/argv0

This patch adds explicit support for setting the value of `argv[0]`
when spawning a process.

PR-URL: https://github.com/nodejs/node/pull/7696


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>