logging: support an "OFF" logging level

logging: support an "OFF" logging level

The only way to disable logging for a domain entirely is to
omit the domain from the "domains" list. For example:

  "level=INFO, domains=PLATFORM,..."

Now add an explicit level "OFF" to facilitate configuration like:

  "level=INFO, domains=ALL,WIFI_SCAN:OFF"

It also supports
  "level=OFF, domains=PLATFORM:INFO"
but this is for the most part equivalent to
  "level=INFO, domains=PLATFORM"