Linkchecker: option to override user agent
linkcheck.py currently hardcodes a 'Mozilla/5.0' user agent to simulate a browser, which works with most sites.
But Sourceforge resets the connection for that particular string. Interestingly enough, it works OK for other user agents, including 'Mozilla/4.0'.
It may be the case that other websites exhibit similar quirks, and it would be nice if we could specify a string to be used as the user agent in conf.py.