Silently ignore unknown sub-TLVs
Silently ignore unknown sub-TLVs

This is a MUST according to the Babel Extension Mechanism draft
(Section 2.2.3).

dtaht/babeld-shortrtt-metrics,woniullb/babeld,boutier/babeld,Drooids/babeld,jech/babeld,wlanslovenija/babeld,sudomesh/babeld,wlanslovenija/babeld,jech/babeld,Gwendocg/babeldToS,Gwendocg/babeldToS,tcatm/babeld