get_first_struc_idx

get index of first structure type
returns: -1 if no structure type is defined
         index of first structure type.
         Each structure type has an index and ID.
         INDEX determines position of structure definition
          in the list of structure definitions. Index 1
          is listed first, after index 2 and so on.
          The index of a structure type can be changed any
          time, leading to movement of the structure definition
          in the list of structure definitions.
         ID uniquely denotes a structure type. A structure
          gets a unique ID at the creation time and this ID
          can't be changed. Even when the structure type gets
          deleted, its ID won't be reused in the future.

long get_first_struc_idx();