speedtest-netperf: add idle latency measurement

speedtest-netperf: add idle latency measurement

Allow measuring ping latency and CPU details at idle as a baseline before
measuring under data transfer loading. This allows better determination of
Latency Under Load, a critical bufferbloat parameter. The CPU details can
also be used to verify idle conditions or examine CPU frequency against
ping variations and jitter.

Change the default test duration to 30 seconds, which is adequate for SQM
tuning while reducing bandwidth consumption for upstream netperf servers.

Change the default ping host from gstatic.com to one.one.one.one, which is
widely available and generally shows lower latency.

When warning of internal netperf errors, suggest running netperf directly
to view error details.

Other minor updates include:
  - clear tmp file names for safety in case of traps
  - simplify ping code, argument parsing and number validation
  - fix cases of wrong protocol usage with hostname as ping target
  - drop unneeded egrep usage

Also update README accordingly, with clearer usage text and terminology.

Signed-off-by: Tony Ambardar <itugrok@yahoo.com>