bug#42134: icecat: can't go back to duckduckgo search results

[Jonathan Brielmaier]

When you search for something with DuckDuckGo, click on a result and
then click on your browsers back button you end up at DDGs start page
and not the results page.

1. Enter "guix" in your search/address bar while having DDG as default
search engine.
2. Click on first result -> guix.gnu.org
3. Now click after landing at our beautiful website on the browsers back
button (<-)
4. You end up at https://duckduckgo.com/?ia=web and not at
https://duckduckgo.com/?q=guix

This does NOT happen when you
1. use DuckDuckGo in Chromium via it's address bar
2. use DDG in Icecat starting from duckduckgo.com and not the
address/search bar
3. use Bing or Google via Icecat's address/search bar

I have disabled the "Spoof Referers" setting which comes from Icecat and
is available at about:preferences#privacy

Icecat has some custom DDG search plugin:
https://git.savannah.gnu.org/cgit/gnuzilla.git/tree/makeicecat#n172
https://git.savannah.gnu.org/cgit/gnuzilla.git/tree/data/searchplugins/duckduckgo.xml

Maybe that's breaking that. I don't know.