sighandler() should take 2 arguments

Stopping arcstat.py with ^C always ends up with error:

TypeError: sighandler() takes no arguments (2 given)

This patch corrects the error by updating the signal handler
to take the correct number of arguments.

Signed-off-by: Isaac Huang <he.huang@intel.com>
Signed-off-by: Prakash Surya <surya1@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2182