Robustness: wrap any calls to System.getProperty in case security is enabled.

Since calling System.getProperty may throw an exception on locked-down systems.

Note: this patch has the side-effect of making the 'guice_include_stack_traces'
setting static, rather than querying the system property each time a module is
installed. Making the setting static improves performance as System.getProperty
can be a bottleneck, at the cost of being able to change the setting on-the-fly.

If the 'guice_include_stack_traces' setting should be dynamic then I can amend
the patch accordingly.
Refactored the custom class loading option into InternalFlags to be more consistent: a3fe411

Note I didn't add a default value to the `getSystemFlag` method because while in most cases you'd want to return the default when there's a security exception, when it comes to custom class loading it makes more sense to return OFF which is not the default.