EP_ActSp ($21) - Return active file space in slot C

IN:
   A = EP_ACTSP
   C = slot number containing File Eprom Area

OUT:
   Fc = 0, File Eprom available
        DEBC = Active space (amount of visible files) in bytes
               (DE = high 16bit, BC = low 16bit)

   Fc = 1,
        A = RC_ONF
        File Eprom was not found in slot C.

Registers changed after (succesful) return:
   A.....HL/IXIY same
   .FBCDE../.... different