Add burst mode I/O.

CIO calls the N: handler once for each character transferred.

If open (aux1) mode is not update (12), and if a buffer size and length are specified, then call SIO to transfer the desired number of bytes (minus 1) directly to the buffer, reset the buffer length to 1, and then exit, which will cause CIO to transfer the last character itself.

DOS 2.0's implementation of burst mode is described here:
https://www.atariarchives.org/iad/chapter12.php