Add an utility function for resetting addresses/routes of NMIP6Config
from NMNDisc data. For one, this de-duplicates code in device and
nm-iface-helper.

Also, we no longer first reset (delete) all addresses and add them anew.
Instead, we first mark all entries as dirty for deletion, merge (append)
the new entires, and delete the remaining dirty entires. This saves a
extra work, in the expected case where NMIP6Config already contains
several of the new entries.