Twisted system event triggers fix 2007-08-01

One of the older, darker corners of Twisted, system event triggers (part of the core reactor interface) have long had slightly poor handling of some corner cases. Fortunately most use of these triggers is via the application service system which avoids all of these cases, but there is another startup notification API through which one might have encountered some weird behavior. As of now, a bug which could cause a trigger to not be executed has been fixed. Code which triggered this behavior was probably buggy itself, since it was asking the reactor to do something impossible; it will receive a warning from now until we turn it into a real exception, a couple releases from now.