Bug 883254 - Add the duckduckgo searchplugin. r=margaret, a=lmandel
Bug 883254 - Add the duckduckgo searchplugin to certain locales. f=glandium, r=mfinkle, a=lmandel

The purpose of this patch is to union a set of extra search plugins into
the set of search plugins for a specific subset of locales in a
multi-locale build.  This is a temporary hack for a single release; for
this release, we don't care about burning single-locale repacks.

I don't think we support finding a localized JAR resource --
like (%searchplugins/list.txt) -- from the object directory directly, so:

* I made the list filename depend on the locale;
* generated the locale-dependent list at build time;
* had the generated JAR include the list from the object directory.