MEDIUM: dns: add a "resolve-net" option which allow to prefer an ip in a network

MEDIUM: dns: add a "resolve-net" option which allow to prefer an ip in a network

This options prioritize th choice of an ip address matching a network. This is
useful with clouds to prefer a local ip. In some cases, a cloud high
avalailibility service can be announced with many ip addresses on many
differents datacenters. The latency between datacenter is not negligible, so
this patch permitsto prefers a local datacenter. If none address matchs the
configured network, another address is selected.