tools/power turbostat: end current interval upon newline input

In turbostat interval mode, a newline typed on standard input
will now conclude the current interval.  Data will immediately
be collected and printed for that interval, and the next interval
will be started.

This is similar to the recently added SIGUSR1 feature.
But that is for use by programs, while this is for interactive use.

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