Various Other changes to get Expt Modify ready for prime time.

 - If assign fails on a modify, experiment will
   be restored to old state, *not* swapped out.

 - Reboot option has been improved to reboot all
   nodes as part of os_setup, not in separate
   step.

 - Different assign error codes result in different
   retry behavior for assign_wrapper
   (Follow's Rob's change to assign to make it
    pass back special code for non-retriable faults)

 - '64' bit in assign_wrapper exit code indicates to tbswap
   that db/phys state hadn't been mucked with before
   the exit occurred
   (ergo, '65' and '1' are the common return codes,
    though the old 4,8,16,32 are still there for assign failing.)

 - (tbswap still returns codes from assign wrapper)

 - Added 5 sec pause between assign attempts.

 - Cleaned up tbswap code.

 - Physical state backup/restore removed from tbprerun,
   put into swapexp.

 - Interfaces table now getting cleaned up correctly
   (Mike noticed problem)

 - Changed menu display in showexp to show
   the "modify" menu option for swapped out experiments
   (like it used to.)

 - A couple other changes.

Note:
 Still admin-only, but I plan to change that soon.

To do:
 - Erase expt backups in /tmp after using them.
 - Re-viz failed experiments.