This function is used to fill in default values in the layer and allocate and
fill the MAP structures in layer. The LayerName field is filled
using DFLT_LAYER_TEXT and layernames (see SHAREDNET-constants). The
LayerId field is given the correct value using the layer_count counter (see
SHAREDNET-variables). NumMaps is copied from the recipe, and the
l and p fields of the UserData and SystemData members are set to 0 and
NULL, respectively.
Finally, all maps are allocated and filled using the create_map routine,
links to the linked list are given in the FirstMap and LastMap field and
links to the linked list of units are given in FirstUnit and LastUnit.NumUnits is set to the right number of units in the layer.
