Delegate Security Manager for Exit Guard

[redcatbear]

Situation

Currently the implementation of the SystemExitGuard extension replaces the existing security guard with a specialized one that intercepts the System.exit. This one has now special permissions and it also has no relationship to a pre-existing one. Other than saving it and restoring it later that is.

What would be less invasive is if the replacement took the existing one as delegate and delegates all checks but the exit check to the original one.

Acceptance Criteria

• ExitGuardSecurityManager delegates all checks to the preexisting one (with exception of the intercepted system exit.
• If no security manager was set, then all checks are granted.

[redcatbear]

Closed with #24.