This function checks, for all UNIT structures in net, if its LINK structures
have valid values.
Checks include: the number of weights is counted and compared to the NET
structure's NumWeights field. Furthermore, the SystemData.l field is set
to FALSE for each weight, after which it is set to TRUE by looping over all
units and traversing their linked list of InLinks and OutLinks. If there
remains a weight with SystemData.l set to FALSE, the weight is not used
(an error).