A pointer to the first of a linked list of LINK structures if
successful, NULL in case of an error.
FUNCTION
This function is called by fscanf_unit twice, to read all incoming
and outgoing links. The link part for each unit is the part between
LINKBEGIN and LINKEND.
See NET-IO-format for the layout of this part.
NOTE
Since at the time of reading a LINK structure not all units are read yet,
it is not possible to fill the Unit field. Therefore, the SystemData.l
field (see system_data) is filled with the unit's ID. The same goes
for the pointers to the weight structures; the weight's ID is stored in
the UserData.l field.
After reading
all units, all unit and weight references in the LINK structures are filled using
the get_unit and get_weight_byid functions.
