Update dyndns.class

Only use IPv4_only function if ipv6 is false
we actually use xmlrpcbaseurl here, not product_website
fix text, remove product_website mentions here since that's not actually used anyway.
use xmlrpcbaseurl here too, not product_website
set package URL to https://packages.pfsense.org
fix plugin method for packages to allow for multiple plugins in an 'item' array as 'plugin' is not allowed as array item by xmlparse.inc
Update dyndns.class
Merge pull request #1026 from PiBa-NL/plugin_array_item

fix plugin method for packages to allow for multiple plugins in an 'item' array as 'plugin' is not allowed as array item by xmlparse.inc
Merge pull request #1025 from florian-asche/patch-5

Update dyndns.class
Provide upgrade code after changes done for Ticket #3441
Fix #3521, show correct field descr
Correct variable name, while here unset some large var
Unbreak listtags(), it should return regular array
Make the parser more efficient to test for keys rather than searching inside the array
Make the Additional tags that may be supplied compliant with new schema