[core] fix fd leak when using libev (fixes #2761)

[core] fix fd leak when using libev (fixes #2761)

server.event-handler = "libev" would leak fds after the change made
in commit:40f16d5 since fdevent_libev.c:fdevent_libev_poll() always
returned 0 ready events after handling the event callbacks itself.
Therefore, fdevent_libev.c:fdevent_libev_poll() must also call
fdevent_sched_run() to handled fds scheduled to be closed.

This bug was introduced in 1.4.42.

(thx mittwinter for troubleshooting and identifying problem)

Note: server.event-handler = "libev" is no longer recommended.
lighttpd provides event handlers optimized for modern systems for most
platforms, and the limited way that lighttpd uses libev does not provide
any advantages over the OS-specific optimized event handlers.

x-ref:
  "fd leak with libev in 1.4.42"
  https://redmine.lighttpd.net/issues/2761