Node.js is currently considering printing a warning when a non-existent
property of `module.exports` is accessed while in a circular `require()`
dependency, in order to make it easier to catch issues with circular
dependencies.

In order to avoid printing these warnings for shelljs, checking for the
property’s existence rather than its truthiness suffices.

Refs: nodejs/node#29935

This locks down file permissions used by the internal implementation of
`shell.exec()`.

Issue #1058
Tested manually using the documented scenarios