gcf2: improve error handling and do not ->fail on wbuf
For historical reasons, both Danga::Socket::write and
PublicInbox::DS::write will return 0 when data is buffered;
so Gcf2Client must not call ->fail when DS::write returns 0.
We'll also improve robustness by recreating the entire
Gcf2Client object if it does die for other reasons, instead of
risking mismatched fields due to deferred close.
We also need to ensure we only get one EPOLLERR wakeup and
issue EPOLL_CTL_DEL if ->event_step is triggered by a dying
Gcf2 process, so always register the FD with EPOLLONESHOT.