utils_posix.py: make sure we create pidfile with standard umask
js/results.js: be robust when raw's web100_snap is {}

Since {} is the default value for previous versions of Neubot, the
code should be able to deal with it correctly.

To verify this diff works, I temporarily modified the raw test
to store {} (instead of []) in the web100_snap field.
test/raw.json: disable average-cwnd and plr plots

In the next release of Neubot, we are not yet ready for this
two plots. I will revert this patch after the release.