EP_FreSp ($24) - Return free file space in slot C

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

OUT:
   Fc = 0, File Area available
        DEBC = Free space available
               (DE = high 16bit, BC = low 16bit)

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

Registers changed after (successful) return:
   ......HL/IXIY same
   AFBCDE../.... different