NAME
    CMD_STOP -- stop device processing (like ^S)

FUNCTION
    CMD_STOP is a standard command for multiple audio channels.  For each
    selected channel (io_Unit), if the allocation key (ioa_AllocKey) is
    correct, CMD_STOP immediately stops any writes (CMD_WRITE) in
    progress; otherwise, CMD_STOP returns an error (ADIOERR_NOALLOCATION).
    cmd_write queues up writes to a stopped channel until cmd_start starts
    the channel or cmd_reset resets the channel. cmd_stop is synchronous
    and only replies (mn_ReplyPort) if the quick flag (IOF_QUICK) is
    clear.  Do not use CMD_STOP in interrupt code at interrupt level 5 or
    higher.

INPUTS
    mn_ReplyPort- pointer to message port that receives I/O request after
                  if the quick flag (IOF_QUICK) is clear
    io_Device   - pointer to device node, must be set by (or copied from
                  I/O block set by) opendevice function
    io_Unit     - bit map of channels to stop (bits 0 thru 3 correspond to
                  channels 0 thru 3)
    io_Command  - command number for CMD_STOP
    io_Flags    - flags, must be cleared if not used:
                  IOF_QUICK - (CLEAR) reply I/O request
    ioa_AllocKey- allocation key, must be set by (or copied from I/O block
                  set by) opendevice function or adcmd_allocate command

OUTPUTS
    io_Unit     - bit map of channels successfully stopped (bits 0 thru 3
                  correspond to channels 0 thru 3)
    io_Error    - error number:
                  0                    - no error
                  ADIOERR_NOALLOCATION - allocation key (ioa_AllocKey)
                                         does not match key for channel