ddns-scripts: allow disabling address binding

[alaviss]

Maintainer: @feckert
Environment: OpenWrt SNAPSHOT r23578-e9911f10e4 on mediatek/mt7622

Description:

If GNU wget or curl is installed and the address source is network, ddns-scripts will try to bind to the interface whose IP is being used for the update.

However this does not work with DDNS providers that only provide IPv4 portal (eg. duckdns.org) but supports IPv6 addresses since there is no way to reach them via IPv6.

How to reproduce this issue:

1. Install GNU wget
2. Install ddns-scripts
3. Configure ddns for duckdns.org for IPv6

Manually specifying bind_network to an IPv4 network doesn't work as the scripts then try to communicate using the IPv6 link-local address of that network.

A workaround is to remove GNU wget from the device.

[brada4]

You can make /etc/wgetrc /root/.wgetrc to contain single -4 to disable ip6.

[roganlynch]

Fixed this just now - Fix IPv6 to IPv4-only DDNS provider #28063