ddns-scripts: add curl source IP bind fallback

ddns-scripts: add curl source IP bind fallback

Keep the existing cURL bind_network behavior of binding to the
logical device first. This preserves the behavior introduced for
PPPoE and multi-WAN setups where the selected network must also be
used for the DDNS update request.

Some setups can still fail when libcurl binds directly to the logical
PPP device. In that case the transfer may time out even though binding
to the source address of the same network succeeds. This can make DDNS
updates fail repeatedly on affected systems.

Retry cURL transfers once with the network source IP when the
device-bound transfer fails. This leaves the normal path unchanged,
keeps the update request on the same network, and matches the existing
GNU Wget behavior which already binds to the network IP address.

Also document the implicit bind_network default and the cURL fallback
in the sample configuration.

Signed-off-by: Jiang Tengfei <redacted>