Fix data corruption bug

Fix data corruption bug

Received data from the client was written to the read-ahead buffer
from dsi_peek() which caused data corrution.
Fix is: change the DSI command buffer from static 8192 bytes to
an allocated buffer of size DSI quantum and use this buffer in
dsi_write/write_fork. That just requires dsi_writeinit to use
memmove instead of memcpy, because now we use the same buffer.