Add "-k" option to pintos script. When used, pintos will scan the

Add "-k" option to pintos script.  When used, pintos will scan the
serial output for key words like "Kernel PANIC" and, if found,
terminate the run.  This is useful for making sure that things like
triple faults or recursive panics don't waste a lot of time by finally
terminating only upon a timeout that might be as much as 10 minutes.
Add "-k" to the command line used in the test scripts.

Also, pintos now saves and restores the terminal settings across the
call to the simulator.  This is because qemu turns off local echo and
doesn't restore it if killed by a signal.