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