Add interface to pressure_above and pressure_rising_beyond for root host pressure monitoring

[danobi]

It would be nice if pressure_above and pressure_rising_beyond plugins were able to monitor root host pressure.

Interface:

• root host is encoded by '/'. ie cgroup=/ as the arg for the plugins

Code changes:

• In PressureAbove::run and PressureRisingBeyond::run, during cgroup wildcard resolution, if the unresolved cgroup == '/', insert '/' into the list of resolved cgroups. Then special case Fs::readMempressure and Fs::readIopressure to pass /proc/pressure/memory and /proc/pressure/io, respectively, to Fs::readRespressure
  • Make sure to handle /proc/mempressure (no IO pressure for 4.11 kernels, though)