strerror: use sys_errlist instead of strerror on Windows

strerror: use sys_errlist instead of strerror on Windows

- Change Curl_strerror to use sys_errlist[errnum] instead of strerror to
  retrieve the error message on Windows.

Windows' strerror writes to a static buffer and is not thread-safe.

Follow-up to 2f0bb86 which removed most instances of strerror in favor
of calling Curl_strerror (which calls strerror_r for other platforms).

Ref: https://github.com/curl/curl/pull/7685
Ref: https://github.com/curl/curl/commit/2f0bb86

Closes https://github.com/curl/curl/pull/7735