select/cons_tres logic flaw

The previous select/cons_tres logic would under some circumstances
allocate a job more cores/CPUs than requested. One specific example
is a cluster having nodes with 4 cores, 2 hyperthreads each. A
job request for 20 tasks would launch 8 tasks on 8 CPUs on each
of 2 nodes and 4 tasks on 6 CPUs on a third node (i.e. a total of
22 CPUs when only 20 are needed).