cpufreq.chart.py: add more accurate cpufreq_stats-based calculations

cpufreq.chart.py: add more accurate cpufreq_stats-based calculations

There were two major problems with this module:

- The 'cpuN' names weren't accurate. The 'self.paths.sort()' was trying
  to compensate for os.walk() enumerating the CPUs out of order (as any
  directory enumeration will do). Unfortunately the sort() function is
  alphabetical, so it would result in a list of paths like this:

    [
        '/sys/.../cpu0/...'
        '/sys/.../cpu1/...'
        '/sys/.../cpu11/...'
        '/sys/.../cpu12/...'
        ...
    ]

  So the chart for cpu2 would actually map to cpu11's stats.

  This can be corrected by extracting the 'cpuN' value that's already
  inside the path anyway.

- The scaling_cur_freq value is an instantaneous value. It only
  represents the current processor P-state at the time it was read, and
  doesn't account for the other 999ms that netdata wasn't looking at the
  value.

  This can be corrected by using data from cpufreq_stats, which includes
  P-state residency statistics. Note that the values in cpufreq_stats
  aren't always valid (e.g. if the cpufreq governor is set to
  'schedutil', the statistic files exist but are are empty), so we can
  just fall back to the inaccurate scaling_cur_freq method if necessary.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>