child_process: don't fork bomb ourselves from -e

Remove the `-e` argument from process.execArgv in child_process.fork()
to keep `node -e 'require("child_process").fork("empty.js")'` from
spawning itself recursively.

Fixes: https://github.com/nodejs/node/issues/3574
PR-URL: https://github.com/nodejs/node/pull/3575


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>