lib.addresses: Improve IPAddr6 constructor
lib.addresses: Improve IPAddr6 constructor

This adds comments, fixes a bug pointed out by Peter Peresini, fixes
initialization for IPv4-mapped addresses, adds checking for initialization
from a raw address.

noxrepo/pox,pthien92/sdn,pthien92/sdn,noxrepo/pox,MurphyMc/pox,MurphyMc/pox,MurphyMc/pox,noxrepo/pox,pthien92/sdn,MurphyMc/pox,pthien92/sdn,pthien92/sdn,noxrepo/pox,MurphyMc/pox