net/tinc: convert to PLUGIN_enabled() helper
net/tinc: convert to PLUGIN_enabled() helper

This can be useful in the future for core code as we could
call these from the upper layers.  Also deduplicates and
flattens the code a bit.

evbevz/plugins,fabianfrz/plugins,fraenki/plugins,evbevz/plugins,evbevz/plugins,fraenki/plugins,dharrigan/plugins,dharrigan/plugins,evbevz/plugins,fraenki/plugins,fabianfrz/plugins,fraenki/plugins,dharrigan/plugins,dharrigan/plugins,fabianfrz/plugins,fabianfrz/plugins,dharrigan/plugins,fraenki/plugins,evbevz/plugins,fraenki/plugins,dharrigan/plugins,fraenki/plugins,fraenki/plugins,fabianfrz/plugins,evbevz/plugins,fabianfrz/plugins,fabianfrz/plugins,evbevz/plugins