tools/power turbostat: on SIGUSR1: sample, print and continue

Interval-mode turbostat now catches and discards SIGUSR1.

Thus, SIGUSR1 can be used to tell turbostat to cut short
the current measurement interval.  Turbostat will then start
the next measurement interval using the regular interval length.

This can be used to give turbostat variable intervals.
Invoke turbostat with --interval LARGE_NUMBER_SEC
and have a program that has permission to send it a SIGUSR1
always before LARGE_NUMBER_SEC expires.

It may also be useful to use "--enable Time_Of_Day_Seconds"
to observe the actual interval length.

Signed-off-by: Len Brown <len.brown@intel.com>