Use more modern upstream libraries.

* Remove Fhs.vardir - this is now /var/lib/xcp
* Use Xenstore library from ocaml-xenstore
* Use Xenctrl library from ocaml-xen-lowlevel-libs
* Syslog -> Syslog_transitional to avoid conflict with upstream package
* Fix ocamldoc to work with Ocaml 4.00.1
* Use upstream rpclib instead of xen-api-libs fork
* Remove some unused code
* Remove some instances of xiu checks

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>
Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>