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