tevent: Start to fix the epoll backend to support 2 fd events on the same fd correctly.

tevent: Start to fix the epoll backend to support 2 fd events on the same fd correctly.

Add a utility function epoll_add_multiplex_fd() and
a new flag EPOLL_ADDITIONAL_FD_FLAG_HAS_MPX.

This will be called by epoll_add_event() to merge two
fde events with the same file descriptor.

Signed-off-by: Jeremy Allison <jra@samba.org>
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>