Description: Find the handle of a specified screen label
Mapped Command:
FUNCTION __ScanScreens(label)
   local n
   for n := 1 to MaxScreens
      if _screens_[n,6] == label
         retu(n)
      endif                                      // if _handles_[n,8] := object
   next                                          // for n := 1 to MaxHandles
RETURN(0)