libnetwork: bridge network with umanaged mode should allow existing host subnet

[Luap99]

In unmanaged mode it is very likely that the subnet that the user chooses is in use on the host as it is assigned the the bridge interface.

# podman network create --interface-name br0 --opt mode=unmanaged --disable-dns --subnet 192.168.122.0/24  unmanaged-br0
Error: subnet 192.168.122.0/24 is already used on the host or by another config

However that is a common use case so we need to allow it, it is then on the user to ensure there will be no ip conflicts in the sunet with other systems on the host, i.e. they likely should reserve a range for podman to use and then specify it with --ip-range so podman only selects ips from there and does not accidentally assign one that is used by other systems on the local LAN.