Changes to support non-racy NFS exports.
As of FreeBSD 9, the race condition in mountd has been addressed. We automatically (based on the OS running) add the option to mountd (-S) as part of ops-install. There is also a configuration variable, NFSRACY, that we auto-detect and set in elabinelab setup. For regular installs, you need to set this manually in the defs-* file. Note that the setting in the defs file does not affect very much. It is passed to the client-side which uses it in one copy function (used as a fall back for getting the topology file). There are still a couple of places in the Emulab code where handling the race condition is hardwired into the code (most notably, imagezip). These remain.