NAME
    ADCMD_FREE -- free audio channels for allocation

FUNCTION
    ADCMD_FREE is a command for multiple audio channels.  For each
    selected channel (io_Unit), if the allocation key (ioa_AllocKey) is
    correct, ADCMD_FREE does the following:
      . restores the channel to a known state (CMD_RESET),
      . changes the channels allocation key, and
      . makes the channel available for re-allocation.
      . If the channel is locked (ADCMD_LOCK) ADCMD_FREE unlocks it and
        clears the bit for the channel (io_Unit) in the lock I/O request.
        If the lock I/O request has no channel bits set ADCMD_FREE replies
        the lock I/O request, and
      . checks if there are allocation requests (ADCMD_ALLOCATE) waiting
        for the channel.

    Otherwise, ADCMD_FREE returns an error (ADIOERR_NOALLOCATION).
    ADCMD_FREE is synchronous and only replies (mn_ReplyPort) if the quick
    flag (IOF_QUICK) is clear.  Do not use ADCMD_FREE in interrupt code.

INPUTS
    mn_ReplyPort- pointer to message port that receives I/O request
                  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 free (bits 0 thru 3 correspond to
                  channels 0 thru 3)
    io_Command  - command number for ADCMD_FREE
    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 freed (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