* Add libadminmfs.pm with routines for entering/exiting and executing
  commands in, the admin MFS.  Node admin and firewall swapout (see
  below) now use this, the image creation process does not yet.

* Add swapout time hooks for running an admin mode process, likely to
  be used to collect swapout time state.  Currently controlled globally
  by two new sitevars.

* Modified node_admin to use the library and added a "-c <command>"
  option to have nodes go into admin mode and run a command.  I don't
  really expect this to be useful, it was just a testing vehicle for
  the library.

2. Improved the swapout process for firewalled experiments.  Largely
   just generalized what we already did for paniced experiments.
   At swapout, firewalled nodes are:

   - powered off
   - set to boot into admin mode and run a disk zapper
   - powered on

  The swapout process then waits for all nodes to successfully complete
  disk zapage, at which point the nodes are nfree'ed as usual.  Any
  failure of the above process, marks the experiment as panic'ed (to
  ensure that we are involved in cleanup) and sends mail to testbed-ops
  describing the state of the nodes.

3. Added the aforementioned disk zapper, a little C program in the MFS
   which zeroes out the MBR and partition boot blocks (but not the MBR
   partition table or FS superblocks).  This is added insurance that if
   a node somehow gets diverted after being nfree'd but before getting
   the disk reloaded (e.g., goes to hwdown), that we cannot accidentally
   boot from the disk.  This program gets installed in the admin MFS.

4. Related to firewalls, modified swapin to use the new documented
   "snmpit -N" to get the firewall VLAN number rather than parsing the
   output that was a side-effect of VLAN creation.