Addlabel (string, x, y, z)

   Synopsis:
      Adds a new label near the automapper's current room

   Notes:
      'string' is the text to use in the label. The remaining three arguments
         are the coordinates of the gridblock, relative to the automapper's
         current room, at which the label will be drawn.
      If the operation is successful, the new label's number is returned. If
         there is no current room, if 'string' is an empty string, if the
         coordinates x, y and z are invalid or if the operation fails, 0 is
         returned.

   Examples:
      Addlabel ("text", 1, 2, 0)    >> Adds a label one block below and two
                                       blocks to the right of the current room
      Addlabel ("text", -3, -4, 0)  >> Adds a label three blocks above and four
                                       blocks to the left of the current room
      Addlabel ("text", 0, 0, 1)    >> Adds a label one level higher than the
                                       current room
      Addlabel ("text", 0, 0, -2)   >> Adds a label two levels lower than the
                                       current room
