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

Marlin/Marlin_main.cpp

   SERIAL_PROTOCOLPGM(MSG_OK);
   #ifdef ADVANCED_OK
     SERIAL_PROTOCOLPGM(" N"); SERIAL_PROTOCOL(gcode_LastN);
-    SERIAL_PROTOCOLPGM(" S"); SERIAL_PROTOCOLLN(commands_in_queue);
+    SERIAL_PROTOCOLPGM(" P"); SERIAL_PROTOCOL(BUFSIZE - commands_in_queue);
   #endif
+  SERIAL_PROTOCOLLNPGM("");  
 }
 
 void get_coordinates() {