Convert --exclude option parsing to new form.

Change slurm_opt_t field to 'exclude' to match command line.

Node that the _valid_node_list() call is being moved into _opt_verify()
where it probably should have been before. Since _valid_node_list()
depends on several other settings, processing it when --exclude is
processed may have resulted in inconsistent behavior if the order of the
various arguments was changed around.