Linux: Support wireless IP roaming

Linux: Support wireless IP roaming

This is achieved by checking that the interface is wireless,
IFF_UP and IFF_LOWER_UP are present, but IFF_RUNNING is missing.

This gives exactly the same support as modern NetBSD when carrier
loss is detected, but without the address verifications when the
carrier comes back as that needs to be handled in the kernel.

While IP setup is maintained, other configuration data is discarded.
Note that this should be improved in the future.

Thanks to Boris Krasnovskiy <borkra@gmail.com> for helping with this.