Improved manual DNS address addition for dual stack case
Improved manual DNS address addition for dual stack case

If DHCP or PPP does not provide DNS addresses, for dual stack, adds both ipv4
and ipv6 DNS addresses to DNS list.

YarivCol/mbed-os,CalSol/mbed,karsev/mbed-os,Archcady/mbed-os,catiedev/mbed-os,nRFMesh/mbed-os,mazimkhan/mbed-os,andcor02/mbed-os,kjbracey-arm/mbed,mbedmicro/mbed,c1728p9/mbed-os,catiedev/mbed-os,mazimkhan/mbed-os,ryankurte/mbed-os,YarivCol/mbed-os,Archcady/mbed-os,karsev/mbed-os,CalSol/mbed,c1728p9/mbed-os,karsev/mbed-os,HeadsUpDisplayInc/mbed,betzw/mbed-os,nRFMesh/mbed-os,catiedev/mbed-os,andcor02/mbed-os,karsev/mbed-os,infinnovation/mbed-os,kjbracey-arm/mbed,ryankurte/mbed-os,mbedmicro/mbed,nRFMesh/mbed-os,infinnovation/mbed-os,svogl/mbed-os,ryankurte/mbed-os,HeadsUpDisplayInc/mbed,Archcady/mbed-os,CalSol/mbed,mazimkhan/mbed-os,svogl/mbed-os,YarivCol/mbed-os,CalSol/mbed,CalSol/mbed,mazimkhan/mbed-os,mbedmicro/mbed,infinnovation/mbed-os,andcor02/mbed-os,betzw/mbed-os,karsev/mbed-os,c1728p9/mbed-os,kjbracey-arm/mbed,svogl/mbed-os,svogl/mbed-os,HeadsUpDisplayInc/mbed,catiedev/mbed-os,CalSol/mbed,YarivCol/mbed-os,catiedev/mbed-os,HeadsUpDisplayInc/mbed,YarivCol/mbed-os,Archcady/mbed-os,infinnovation/mbed-os,karsev/mbed-os,Archcady/mbed-os,svogl/mbed-os,nRFMesh/mbed-os,infinnovation/mbed-os,andcor02/mbed-os,c1728p9/mbed-os,infinnovation/mbed-os,ryankurte/mbed-os,svogl/mbed-os,betzw/mbed-os,mazimkhan/mbed-os,c1728p9/mbed-os,kjbracey-arm/mbed,ryankurte/mbed-os,HeadsUpDisplayInc/mbed,mbedmicro/mbed,mazimkhan/mbed-os,YarivCol/mbed-os,HeadsUpDisplayInc/mbed,ryankurte/mbed-os,Archcady/mbed-os,andcor02/mbed-os,betzw/mbed-os,mbedmicro/mbed,nRFMesh/mbed-os,betzw/mbed-os,betzw/mbed-os,andcor02/mbed-os,catiedev/mbed-os,c1728p9/mbed-os,nRFMesh/mbed-os