Remove internal separated behavior of the easy vs multi intercace.
curl_easy_perform() is now using the multi interface itself.
Several minor multi interface quirks and bugs have been fixed in the
process.
Much help with debugging this has been provided by: Yang Tse
Verify that the "Saved to filename 'blabla'" message is only displayed when
the 'blabla' filename being used _actually_ has been specified by the server
in the Content-Disposition header.
Use relative path for unintended file creation postcheck.
