When iostat(8) receives SIGINT while running with "-w" or "-c", it will now
print statistics one more time before exiting. Also, it now implements the
wait using setitimer instead of sleep, so the waits will be more consistent
when the system is heavily loaded.

MFC after: 4 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D4473