child_process: add shell option to spawn()

This commit adds a shell option, to spawn() and spawnSync(). This
option allows child processes to be spawned with or without a
shell. The option also allows a custom shell to be defined, for
compatibility with exec()'s shell option.

Fixes: https://github.com/nodejs/node/issues/1009
PR-URL: https://github.com/nodejs/node/pull/4598


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>