Some routers are so broken that UPnP NAT don't report ConflictInMappingEntry
when redirecting the same port several times.

Here is for example what we had with a Numericable Box (France):

0 (1024, 'TCP', ('192.168.0.29', 1194), 're6stnet openvpn server (1194/tcp)', '1', '', 0)
1 (1024, 'TCP', ('192.168.0.16', 1194), 're6stnet openvpn server (1194/tcp)', '1', '', 0)
2 (1024, 'TCP', ('192.168.0.33', 1194), 're6stnet openvpn server (1194/tcp)', '1', '', 0)
3 (1024, 'TCP', ('192.168.0.20', 1194), 're6stnet openvpn server (1194/tcp)', '1', '', 0)
('192.168.0.29', 1194, 're6stnet openvpn server (1194/tcp)', True, 0)

Obviously, this can't work.

It seems that this router also accepts a limited number of NAT rules, far less
than we'd like, so even if there's still a probability of conflict with this
commit, it will be good enough for our use.