--request-target: instead of --strip-path-slash

... and CURLOPT_REQUEST_TARGET instead of CURLOPT_STRIP_PATH_SLASH.

This option instead provides the full "alternative" target to use in the
request, instead of extracting the path from the URL.

Test 1298 and 1299 updated accordingly.

Idea-by: Evert Pot
Suggestion: https://daniel.haxx.se/blog/2017/06/19/options-with-curl/comment-page-1/#comment-18373

Closes #1593