ipv6PrefixChanged.em0 needs method to determine old IPv6 address

[fiwswe]

Currently no direct way exists to get the previous IPv6 address in ipv6PrefixChanged.em0.

However in the scenarios this script is intended for the interface identifier should be stable. So extracting that and combining it with the old IPv6 address prefix, which is known — see variable lastPrefix —, should be possible.

This would be needed for sed(1) replacement of the IP in various config files or for deactivating the old IP in services after the new one has been configured.