Use a custom ifa_rtrequest function for point-to-point interfaces

Use a custom ifa_rtrequest function for point-to-point interfaces
instead of relying on hacks in nd6_rtrequest() to add a route to
loopback for each address configured on such interfaces.

While here document that abusing lo0 for local traffic is not safe
for interfaces in a non-default rdomain.

Tested by claudio@, jca@ and sthen@, ok sthen@