Add experimental kqueue support [1]:

Added support for using kqueue in slapd (for systems that support kqueue(2).

Patch obtained from:

http://public.me.com/bryan.duncan/bryan-duncan.kqueue.090922.patch

1. This patch file is derived from OpenLDAP Software. All of the
2. modifications to OpenLDAP Software represented in the following
3. patch(es) were developed by Apple. I, Bryan Duncan, am authorized by
4. Apple, my employer, to release this work under the following terms. #
5. Copyright 2009 Apple Inc. All rights reserved. #
6. Redistribution and use in source and binary forms, with or without
7. modification, are permitted only as authorized by the OpenLDAP
8. Public License. #
9. A copy of this license is available in file LICENSE in the
10. top-level directory of the distribution or, alternatively, at
11. http://www.OpenLDAP.org/license.html. #

While I'm there, make icu a dependency and conditionally make use
of fdatasync for FreeBSD versions where it is available.

Obtained from: https://github.com/quanah/openldap-scratch/commits/its6300 [1]