Redirect OSDN mail archive searches from DuckDuckGo to Google.

* site.js (osdn_archive, ddg_query, ddg_widget): New functions; they
facilitate setup and initiation of a DuckDuckGo query of any OSDN mail
archive, with redirection via the Google search engine, using...
(ddg_bang, ddg_google_search, ddg_google_search_site): ...these new
internal-use helper functions.

* mailing.html (MinGW-Users, MinGW-Notify): Use...
(ddg_widget, osdn_archive): ...these to initiate DuckDuckGo search...
(ddg_google_search): ...with this to redirect to the Google engine.