Change ident for EVFILT_USER to 0 and add a test (#1582) Conventionally, ident for EVFILT_USER is set to 0 to avoid collision of file descriptors, which is what other renowned networking frameworks like netty(java), mio(rust), gnet(go), swift-nio(swift), etc. do currently. Co-authored-by: Azat Khuzhin <azat@libevent.org>