update content
- linux aio backend almost complete rewritten to work around its limitations.
- epoll backend now mandatory for linux aio backend.
- fail assertions more aggressively on invalid fd's detected in the event loop, do not just silently fd_kill in case of user error.
- ev_io_start/ev_io_stop now verify the watcher fd using a syscall when EV_VERIFY is 2 or higher.