GN_Gdn, Convert an ASCII string to a binary integer number

IN:
     HL = local source pointer
          HL = 0, read from stream IX handle
          HL = 1, read from filter IX handle
          HL = 2, not allowed
          > 255, read string at (HL)
     DE = destination pointer.
          DE = 0, 1 not allowed
          DE = 2, return integer in BC (converted to 16bit
          range)
          > 255, store integer at (DE). (DE) = least
          significant byte, (DE+3)= most significant byte.

     B = maximum number of characters to be read
     IX = source handle (if HL = 0,1)

OUT, if call successful:
     Fc = 0
     Fz = 1, conversion recognised a number
     A = character which stopped conversion
     HL, if HL(in) < 2, indexes conversion terminator
     HL, if HL(in) > 255, then points to conversion terminator.
     BC = result, if DE(in) = 2

OUT, if call failed:
     Fc = 1
     A = error code:
          RC_OVF ($0B), overflow, result invalid
          RC_EOF ($09), reading an empty stream or filter

Registers changed after return:
     ...DE../IXIY same
     AFB..HL/.... different