avr_at86rf
sim: Fix the miss close and unlink pipe file
sim: Fix ctrl + c will result the process could not exit
Sim: Fix the node is not exit for quit comman

When input quit command, The sim will exit,
At the time, the erron is EEXIST. So It could
not to kill the process of all the node

signed-off-by: Zhong Hongbo <bocui107@gmail.com>