Get rid of the sequential access feature of the lists. This was used

Get rid of the sequential access feature of the lists. This was used
only in a couple of places and all of them except for one were easily
converted to use Lst_First/Lst_Succ. The one place is compatibility
mode in job.c where the it was used to advance to the next command on
each invocation of JobStart. For this case add a pointer to the node to
hold the currently executed command.