Incorrect error handling in io_epoll_context

The code in io_epoll_context implies that `readv` and `writev` syscalls error codes are returned as different negative values. Instead, they always return -1, and actual error code is returned through `errno` variable. The only reason it currently works (even though it always adds fds to `epoll` even if there's an unrecoverable error, not just `EAGAIN`, never calling `set_error` in `start_io`, and calling `set_error` with `EPERM` in case of ANY error, hiding it) is because it compares return code with `-EPERM`, which turns out to be -1 on Linux, and adds such descriptors to `epoll`. I wonder where the idea of this check comes from, cause adding such descriptor to `epoll` wouldn't fix real `EPERM` error...