Startup code should honor shutdown flag more efficiently

The startup code in zope.app.server.main.setup() doesn't consider the possibility that the database opened event might trigger application code that wants to stop the server. The exit_status flag should be checked after any operation in setup that might trigger application code. In particular, the servers should not be started if exit_status has been set. (Even opening the databases might cause a shutdown request.)

For example, a generation that fails and can't allow the database to be used for normal request processing may want to shutdown; it should be able to use zope.app.applicationcontrol.interfaces.IServerControl.shutdown(0) to cancel the startup. By calling shutdown(0) and then sys.exit(), the generation will be cancelled and, eventually, the process will exit.