network: Delay addition of IPv6 Proxy NDP addresses

network: Delay addition of IPv6 Proxy NDP addresses

Setting of IPv6 Proxy NDP addresses must be done at the same
time as static addresses, static routes, and other link attributes
that must be configured when the link is up. Doing this ensures
that they are reconfigured on the link if the link goes down
and returns to service.

(cherry picked from commit 12f7469bbe0142d7f360a29ca2b407ce7f5ff096)

Fixes https://github.com/systemd/systemd-stable/issues/89