understanding of how mountd operates.

Things that should be fixed:

1. It iterates over every node calculating what directories are exported,
   what FSes they are on, etc.  Most of that work only needs to happen
   per experiment.

2. The algorithm to determine what FS a directory is on is a hack.  I just
   take the first component of the path provided.  As long as we mount all
   our FSes at the top level and configure with canonical pathes, this is
   ok.  Other solutions require calling out to the ops node to get actual
   mount info.

3. Once shared experiments are revived, the code to determine exported
   directories will need to change.  The algorithm for computing the
   exports lines should still be correct.