1. Don't stat all exported mountpoints from boss to see if they exist.
   Not only is this wrong from boss (a mount may be valid, but not yet
   exported) it also makes the automounter work its butt off as we might
   stat hundreds or thousands of directories across NFS. The original
   purpose of this was to keep mountd from freaking out when presented
   with a non-existent directory. Since mountd freaking out is about as
   rare as the sun rising in the east, we keep this check but do it on
   the proxy (ops) side instead.

2. Have exports_setup (boss) generate a list of directories that it should
   be able to mount. This goes in /usr/testbed/etc/validmounts.txt and will
   be used by autofs (I hope) to narrow down the wildcarded "/users/*"
   and "/proj/*" mounts to those that might actually work. Right now you
   can "ls /users/WtF" and it will try to mount it from ops and take 4
   seconds to timeout and fail.