Fix issue with --continue and -O are passed together

This set of commits fixes a regression in the behaviour of Wget2.

Wget 1.x has for a long time supported using -c and -O options together in an undocumented manner. This behaviour is now both accepted and expected behaviour. Hence, update how Wget2 handles such a corner case.

The documentation for Wget2 has also been updated to reflect the fact that Wget2 will not consider -O to be merely a shell redirection. This "feature" of Wget 1.x has in the past caused plenty of confusion and should be simply avoided. Instead, the behaviour of -O in Wget2 will be explicitly documented.