server: Create an inproc sync for user APC signaling.
Unlike the message queue for which I believe we can get rid of any special handling, I don't see a way to avoid a dedicated inproc sync for user APCs. This will make sure that the sync gets created on thread creation. It could arguably be lazily allocated instead, but I don't see a reason to delay the failure to a later time and end up with successfully created but unalertable thread when they are supposed to be.