OFKernelEventObserver_kqueue: Ignore EBADF

Removed file descriptors were returned in the event list with EBADF if
the file descriptor has been closed before removing them. Interpreting
that as a read event caused problems, as the file descriptor could not
be mapped to an object anymore, thus the entry in the event list is
skipped now if there was a EBADF error.
OFUDPSocket: Conform to OFCopying.