ddclient: T5144: Fix migration to avoid config name conflict

When migrating from service dns dynamic interface <interface> ... to
service dns dynamic address <address> ..., the config name can
potentially have a conflict when address == 'web'.

Although the /run/ddclient/ddclient.conf that was generated earlier
was incorrect, one could still potentially have misconfigured VyOS
config without realizing it.

We now append the old <interface> name to the config name to avoid
conflict.