Some fixes for robustness (make sure variables exist before using them

Some fixes for robustness (make sure variables exist before using them
because it flags errors when in debug mode).

Also, when specifying array values by name use quotes around the name

 eg  $array['value'] instead of $array[value]

Finally, I changed the way user restoring works (slightly).  Before, if a
restored user was found to be a course creator or an admin, then this
status was restored.  I don't think this is a good idea - it could lead
to security problems when copying a backup from one site to another,
or if that person's status has changed since the backup was made.
I think it's best to only restore what is actually part of the course,
ie teacher or student.

If this broke anything else let me know!