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.