Renovate syslog_ng_stats

Make a number of improvements to the syslog-ng stats plugin:
- Add ctl_path environment variable for location of syslog-ng-ctl
- Remove dependency on List::MoreUtils
- Add default filters for stamp type and dynamic/orphan states
- Generate field names using all source identifiers (avoiding duplicates)
- Use clean_fieldname on field names
- Generate labels in the form that syslog-ng-ctl stats prints

Note that, since cleaning the field names and avoiding duplicates changes
existing field names, this version is not compatible with data from the
previous versions of this plugin.