sighandler() should take 2 arguments

Stopping arcstat.py with ^C always ends up with error:
TypeError: sighandler() takes no arguments (2 given)

Since no special signal handling was done in sighandler(),
it's simpler to just set SIGINT handler to SIG_DFL, which
terminates the script.

Signed-off-by: Isaac Huang <he.huang@intel.com>
Issue #2179