Function ModData(Name:String;Number:Word;Data:Pointer):Word;

    This searches  for  an  item  in  the  directory  matching  Name  and
    Number.  If  found  it  replaces  the  data  in the database with the
    new data specified.   Note  that  the  data  size  must  be the same.
    To change the size  you  must  delete  the  item  and  then insert it
    again.

    The user must ensure 10 kb heap is available.