gcode_process.c: added stop message when M2 is recieved.

This shall help stopping a simulator when G-code is done.
run-in-simulavr.sh: terminate simulation if stop recieved.

Note by Traumflug: this

  while read -r LINE; do
    some commands
  done << (some other command)

didn't work here (bash 4.3.11 on Ubuntu 14.04), so I had to swap
the sequence of these two commands for using a pipe. Anyways,
excellent idea, shortens some simulator runs drastically!