Cleanly exit IOC when stopping IOC execution

Currently, stopping an IOC only terminates the IOC process, but does not allow for that IOC to gracefully shutdown. In some cases, it would be preferable to issue the 'exit()' command at the EPICS shell prior to forcefully terminating the process.

In my experience this is necessary for certain IOCs which are controlling hardware, e.g. camera hardware. I have a concrete example of a failure we experience at SNS. A camera IOC makes a connection to the SDK/API supplied as a shared library by the camera vendor (ANDOR). If the connection is not closed properly, the next attempt to connect to the camera will fail. Issuing the exit() command at the EPICS shell would allow the opportunity for the IOC to 'clean up' by closing the camera instance properly via the vendor API.