IN:
     HL = 0,1,2; not allowed (B=0)
     BHL = extended pointer to filename. HL must be > 255
     (B = 0 -> HL is local pointer)

OUT, if call successful:
     Fc = 0
OUT, if call failed:
     Fc = 1
     A = error code:
          RC_ONF ($12), file not found
          RC_IVF ($17), invalid filename
          RC_USE ($15), file is in use, ie. not closed

Registers changed after return:
     ..BCDEHL/IXIY same
     AF....../.... different