ddns-scripts: Respect config option 'ip_source'

ddns-scripts: Respect config option 'ip_source'

Option 'ip_source' was silently ignored during public IP discovery.

Discovery (in spite of chosen 'ip_source') was based only on
'ip_network', 'ip_interface', 'ip_script' or 'ip_url' options (in this
order) if they were set. This could lead to misleading log entries
"Detect local IP on '$ip_source'" pointing to source that wasn't really
used.

Now only option relevant to configured 'ip_source' is taken into
account.

Signed-off-by: Jacek Politowski <dev@jpol.net.pl>