tools/power turbostat: Use strtoul() for iteration parsing

tools/power turbostat: Use strtoul() for iteration parsing

Replace strtod() with strtoul() and check errno for -n/-N options, since
num_iterations and header_iterations are unsigned long counters. Reject
zero and conversion errors; negative inputs wrap to large positive values
per standard unsigned semantics.

Signed-off-by: Kaushlendra Kumar <kaushlendra.kumar@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>