curl: Build with support for wide-character filesystem APIs on Windows

Curl 7.71 added an option to open files on Windows using wide APIs.  See
curl commit `ffdddb45d9` (curl_setup: support Unicode functions to open
files on Windows, 2020-01-02, `curl-7_71_0~148`).  When enabled, curl's
own narrow APIs expect UTF-8 because that is what their command-line
tool uses internally.  See curl commit `9e5669f388` (tool: support
UTF-16 command line on Windows, 2019-04-12, `curl-7_71_0~149`).

Revert our previous workaround from commit e63dcb1378 (Encoding: Use
encoding libcurl expects with file: urls, 2014-11-05, v3.2.0-rc1~420^2)
in favor of passing URLs to curl encoded as UTF-8.