child_process: set shell to false in fork()

This commit ensures that spawn()'s shell option is unconditionally
set to false when fork() is called.

Refs: nodejs/node#15299
Fixes: nodejs/node#13983
PR-URL: nodejs/node#15352
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Richard Lau <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>

Author: Alex Gresnel