GN_Gtm, Convert an ASCII string to a time in internal format

IN:
     HL = source pointer
          0 = read from stream IX handle
          1 = read from filter IX handle
          2 = not allowed
          >255 = read string at (HL)

     DE = destination pointer
          0 = not allowed
          1 = not allowed
          2 = result returned in ABC
          >255 = store result at (DE)
     IX = source handle (if HL = 0,1)

OUT, if call successful:
     Fc = 0
     HL, HL(in) < 2, HL(in)
     HL, HL(in) > 255, HL points to terminating character
     ABC, DE(in) = 2, result
     ABC, DE(in) > 255, ABC(in)

OUT, if call failed:
     Fc = 1
     A = error code:
          RC_SNTX ($0C), Bad syntax

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