Disallow of feature flags to use licensed names

This ensures that all feature flags are explicitly
used, and have a unique meaning.

This work does:
- hide `licensed` from `bin/feature-flag`
- discovers the feature flag names to collide with licensed feature
  names
- disallows to use licensed names in feature flag API
  (unless `force=1`)
- removes mentions about implicit feature flags

This does not:
- remove implicit feature flag check
- the removal of implicit feature flag will be done at some point later