Gres accounting enhancements

I performed more testing today and for the most part things
worked except for one glitch.  In the case of batch jobs, the GRES
req field was being lost.  It was not being uploaded into the DB and
it was missing from "scontrol show job ...."  This didn't occur with
interactive jobs.  After looking at this, it appears that we enter
select_nodes twice for the batch jobs and only once for interactive
jobs.  Consequently, we call _fill_in_gres_fields twice for the
batch jobs.  On the first entry, the job_ptr->node_cnt entry is 0.
Thus we don't perform the computation and append the string to gres.
Because we are now using the same gres string field, what this
means is that we tokenize the gres string and then don't rebuild
it--thus it becomes blank.

One solution is simply to comment out the second clause of the
if-statement where it checks whether the node_cnt > 0.  This works
because it doesn't matter if on the first pass there is something
like "gpu:0" in the gres string (0 being due to an initial node_cnt
of 0) because we just need the type names of this string.  We use
that to extract from the gres_list the actual value requested.  Thus
as long as we have at least rebuilt the string to contain the type
names, on the subsequent entry into _fill_in_gres_fields, when we do
have the correct node_cnt value, the correct string will be built.
     A better solution is to avoid the entire block of code where we
do the tokenization of the gres string if node_cnt is 0.  This way,
we would not even tokenize the string on the first entry and avoid
some double work.  Being that we enter twice for batch jobs, I also
placed a condition around the the gres_alloc building part of the
function so that we only attempt to build that string when there
isn't already a string there (again avoiding double duty).