Increase serial line buffer

[Guy Harris]

>Can anyone tell me the way to do it?

You get the source to ".../sys/sundev/zs_async.c", change ZSIBUFSZ from
256 to some larger value, change the indices into the software silo from
"u_char" to some data type large enough to hold the new maximum value,
and add code after the code that bumps the indices to take them modulo
ZSIBUFSZ when they overflow ZSIBUFSZ.

Is the buffer overflowing (as indicated by "zsN: ring buffer overflow"
messages)?