[fix] startpage engine - avoid captcha

Startpage has introduced new anti-scraping measures that make SearXNG instances
run into captchas:

1. some arguments has been removed and a new `sc` has been added.
2. search path changed from `do/search` to `sp/search`
3. POST request is no longer needed

Closes: https://github.com/searxng/searxng/issues/692


Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>