Instead have one particular nm_ip6_config_get_address_first_nontentative() function,
make it more extendable. Now, we pass a match-type argument, which can control which
element to search.

This patch has no change in behavior, but it already makes clear, that
nm_ip6_config_get_address_first_nontentative() was buggy, because it would
also return addresses that failed DAD.