nlpacket: use IFA_LOCAL if exist

[aderumier]

we should use IFA_LOCAL first if it's exist.
Currently, when a peer is defined (with pointopoint option for example)
the IFA_ADDRESS is replaced by the peer address

ipv6 don't have IFA_LOCAL by default, only when a peer exist

ipv6 with peer

auto eno4
iface eno4
address 2a0a:1580:2000:100::e/64
pointopoint 2a0a:1580:2000:100::1

{   'attrs': [   ('IFA_LOCAL', '2a0a:1580:2000:100::e'),
                 ('IFA_ADDRESS', '2a0a:1580:2000:100::1'),

ipv6 without peer

auto eno4
iface eno4
address 2a0a:1580:2000:100::e/64

{   'attrs': [   ('IFA_ADDRESS', '2a0a:1580:2000:100::e'),

ipv4 with peer
iface eno3
address 192.168.0.2/32
pointopoint 192.168.0.254

{   'attrs': [   ('IFA_ADDRESS', '192.168.0.254'),
                 ('IFA_LOCAL', '192.168.0.2'),

ipv4 without peer

iface eno3
address 192.168.0.2/32

{   'attrs': [   ('IFA_ADDRESS', '192.168.0.2'),
                 ('IFA_LOCAL', '192.168.0.2'),

[svenauhagen]

@aderumier I just saw your PR.
I stumbled upon that as well on a PPPoE connection, IFA_LOCAL should always be used first and IFA_ADDRESS second if not set as a general rule.
@julienfortin any way you can have a look at the PR?