CURLM_CALL_MULTI_PERFORM stopped being a valid return code from
curl_multi_perform back in 7.20.0. All the libcurl tests are ajusted to
this and no longer check for this return code. Makes them simpler.
All C and H files now (should) feature the proper project curl source
code header, which includes basic info, a copyright statement and some
basic disclaimers.
This test was added after the HTTPS-using-multi-interface with OpenSSL
regression of 7.19.1 to hopefully prevent this embarassing mistake from
appearing again... Unfortunately the bug wasn't triggered by this test, which
presumably is because the connect to a local server is too fast/different
compared to the real/distant servers we saw the bug happen with.