advanced 'ok'

ok N(linenumber) P(bufferremaining)
the host can use this to send the data much faster.

Some pseudo code:
If (bufferremaining > 0)  send command
## as long there is free buffer send command

Also
if (linenumber > lastsend - bufferremaining) send command
## as long last linenumber received is bigger than lastsend (host) - last bufferremaining received