net: add get_local_ip helper

net: add get_local_ip helper

Sometimes we need to have a fallback for gai (get_ip_from_hostname)
but cannot yet rely on configured networks (get_reachable_networks)
from kernel POV (those may not have been configured yet, e.g., on
boot), so the ones configured in /etc/network/interfaces would be
nice too then, as they're the ones that will get configured soon
anyway on boot.

Add a new helper that takes in all those sources and allows to return
a single (first found) or all of those addresses.

Still prioritize the address we get from getaddrinfo, as there the
admin has control through /etc/hosts, DNS and gai.conf and treat the
remaining ones as fallback.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>