Fix merging of interfaces in add_ifconf.
Fix merging of interfaces in add_ifconf.

This avoids multiple config file entries for the same interface
causing an assertion failure.  Reported by Baptiste Jonglez.

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