Use epoll_ctl_batch

We could remove the for-loop doing a series of EPOLL_CTL_ADD operations if we used epoll_ctl_batch().

The for-loop: https://gitlab.com/emilua/glib/-/blob/7b03bd70b487fe8cfa80fbe7f0fcc20e1ec6a5c8/src/service.cpp#L192

The epoll_ctl_batch() syscall: https://lwn.net/Articles/633422/

Unfortunately the epoll_ctl_batch() effort seems to be stalled, so we can't really use it. Therefore this issue. If that syscall ever goes mainline there's this reminder issue.